The Differences Between Chronological Age and Biological Age

The differences between chronological age and biological age are multiple, starting with the fact that they're quantified differently.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 16 April, 2024

The term “age” is present and implicit in the worldview of human beings, as we’re all characterized by a number that’s increasing with each passing year, and we know that the passage of time is slow but inexorable. In any case, the way to “measure” the passage of time is key when it comes to understanding this concept: Do you want to know the differences between biological age and chronological age?

Although it seems strange, what the calendar says isn’t doesn’t always match up with what’s going on inside the body of a human being. If you want to learn more about the differences and the markers that indicate each type of age, we recommend that you continue reading.

What is age?

Before we dive into the differences between chronological age and biological age, it’s important to understand a bit about what the term “age” means in general. According to the Royal Academy of the Spanish Language (RAE), this concept is defined as “the time that a person or another living being has lived.” In addition, age refers to each of the vital stages of the human being (youth, adult, elderly).

Age is synonymous with aging because, with the passage of time, the living tissues of the body begin to wear out. At the microscopic level, aging is related to the degradation of tissues, that is, cells stop dividing and aren’t able to repair those that are already damaged. From a population point of view, aging refers to the percentage of elderly people.

Types of ages

Human beings are guided by chronological age. We’re born one day, and from there, we add intervals in order to quantify the amount of time we’ve been on Earth. In any case, it should be noted that there are several types of ages that don’t respond to the clock. Professional sources highlight the following variants within this term:

• Chronological age: As we’ve said, chronological age is measured in years and includes the time that passes from the birth of the individual to the present.
• Biological age: Refers to the current situation of the person in regard to their potential life cycle. We’ll look at this term in more depth in later paragraphs.
• Psychological age: This is the current individual situation taking into account the individual’s ability to adapt to the environment and certain traits. It’s also characterized by a series of patterns common to the age group to which the human belongs. Put simply and quickly, a 65-year-old isn’t only older, but feels older.
• Social age: This is the age marked by economic, work, and family circumstances (that is, everything that implies living in society). It’s quantified by the individual’s work capacity, the protection of certain groups, and productivity. A 50-year-old has a higher “social age” (and chronological age) than a 13-year-old, so more is expected of them.
• Functional age: This parameter refers to the individual capacity to carry out the demands imposed by a person’s age. This depends on the social, biological, chronological, and psychological variants.

As you can imagine, advancing chronological age also implies a proportional change in all other variants. In any case, the key to this issue is that not all ages advance at the same rate, nor can they be measured in the same way.

What are the differences between chronological age and biological age?

Chronological age responds to an invariable time interval between people (although it sounds redundant, everyone who’s 22 years old has been alive for 22 years). Below, we’ll see that biological age is a much more diffuse term at a medical level than the previous one. We’ll present the differences between both concepts.

1. Biological age isn’t only determined by the ticking of the clock

It’s easy for even the most inexperienced eye to recognize that not all human beings age the same way. A 56-year-old who has taken care of themself all their life may look 40, while a 35-year-old chronic smoker will have the wrinkles expected in a 50-year-old person. For this reason, biological and chronological age aren’t synonymous.

Therefore, one of the main differences between chronological age and biological age is that in the calculation of the latter, the internal functional status and aging of cells are taken as a factor. On the other hand, the chronological variant only refers to the clock: 30 years is 30 years, and they pass by independently of individual traits, as we all live in a specific space-time.

Parameters that define biological age

So, what defines biological age? To calculate it, the following parameters are taken into account:

• Chronological age: It’s impossible to disprove that tissues degrade over time. Therefore, the past time interval is still useful for calculating biological age.
• Genetics: Unfortunately, a person with a genetically acquired disorder (such as Duchenne muscular dystrophy) can have serious physical impairments that aren’t expected in their chronological age. In other words, genes set the rate at which the body breaks down.
• Lifestyle: Being healthy is synonymous with youth, as traits such as obesity are known to act as triggers for premature aging. Exercising and staying active doesn’t ensure full survival in all cases, but it does help.
• Nutrition: As the old saying goes, we are what we eat. For example, there are certain processed meat products confirmed as carcinogenic by the World Health Organization (WHO), which exemplifies that our diet can promote (or prevent) aging or dying before our time.
• Diseases and other conditions: Chronic illnesses are often linked to premature aging, either systemic or of a specific organ.

As you can see, a person’s chronological age is determined entirely by the clock, but the clock is only one of the variants that must be considered when talking about biological age. In any case, and despite the differences, it’s clear that both terms are interconnected.

2. The biological age may be different for each organ

Another difference between biological age and chronological age lies in its “extension” as far as the organism is concerned. Chronological age is global, as it’s assumed that (almost) every cell line in the body has already originated when we’re born, despite the fact that cells are replaced over time. Although this isn’t entirely accurate, in general, we don’t say that “your hair is six months old and your neurons are 33 years old.”

As the Genotypy portal indicates, if we speak in terms of biological age, we can say that a person has a 36-year-old heart and a 58-year-old liver in the same body. As this parameter refers to the degradation of the tissues and the cells that compose them, it’s not surprising that the rate of senescence is different for each organ according to environmental stressors.

Let’s take a look at a practical example. If a person has been drinking alcohol every day for the last 10 years, they’re sure to have liver cirrhosis of varying severity. Every time the liver is injured, it tries to repair itself by forming scar tissue. As collagen fibers are deposited in the organ (fibrosis), it loses functionality.

Due to deterioration, it can be assumed that a person may have a chronological age of 40 and, at the same time, have a liver with a biological age of 70. The biological age of the rest of the tissues is also usually out of step with the liver, but it’s only a matter of time before the failure becomes systemic and the whole body begins to feel the effects of the disease.

3. The markers are different in each case

In biology, the term “marker” is used to designate a segment of DNA at an identifiable position within the genome of a living being. However, there are chemical markers that can be proteins, peptides, and many other complex molecules that aren’t directly in genes.

The only possible “marker” to quantify chronological age is the calendar. Time alone dictates how long a person has been alive (as redundant as it sounds), and this happens in the same way for everyone. In any case, it should be noted that biological age is determined by various markers that have little to do with the day and time. We’ll explain in more detail below.

Telomere length

As the National Human Genome Research Institute indicates, the term “telomere” refers to the end of a chromosome. These structures are repetitive sequences of non-coding DNA in the genome that protect chromosomes from any damage within the cell nucleus. Every time the cell divides, the telomeres of its chromosomes shorten.

Over time, chromosomes are damaged by the shortening of telomeres, and the cell line dies, thus preventing tissue repair. This mechanism explains much of human senescence, so telomere length is a good predictor of how long a cell type has left to live in the body.

It’s interesting to know that there’s an enzyme (telomerase) that’s capable of repairing telomeres, but this is only active in germ lines and stem cells. Even more surprising is that this enzyme is expressed in 75-80% of tumor lines when it shouldn’t, which is why it has been associated with the presence of cancer outside the developmental stages.

Glycans

Glycans are polysaccharides that are found naturally on the surface of cells. The variation of three specific glycans seems to explain 58% of the differences in chronological ages between populations and individuals, as indicated by studies. This biomarker is very novel, so a comprehensive study is still required.

Metabolic rate

The term “metabolic age” refers to the energy expenditure of a human at a specific moment in his life. It’s to be expected that a 70-year-old person will exercise less than a 25-year-old, but this parameter doesn’t only take into account the amount of energy burned by physical activity.

What matters here is the basal metabolic rate (BMR), that is, the amount of energy burned by the human being only by existing (in breathing, thinking, digesting, and other processes). This value represents 60-70% of the daily energy expenditure of an adult individual and varies greatly according to age. It’s not that the elderly exercise less (which also happens), but that their metabolism slows down.

In the elderly, there’s a natural slowing down of metabolism, which translates into a lower basal metabolic rate, weight loss (in general), and functional difficulty at the systemic level. Therefore, the amount of energy burned is a good predictor of the exact state of health of a human being beyond the time they’ve been on Earth.

Chronological age and biological age: Two sides of the same coin

The differences between chronological age and biological age are multiple, but they can be summarized in a single central idea. The chronological variant only takes into account the passage of time, while biological age factors the passage of time, genetics, health, diet, activity, and much more.

Biological age is much more useful than chronological age at a medical level, as it indicates with greater certainty the time that a cell group (or the entire system) has left before collapsing. In addition, it shows that each of the cells “feels” inside the effects of a harmful lifestyle, despite how much we can’t see it.

Obesity, smoking, alcoholism, and many other harmful events drastically reduce the biological age of certain tissues, which always ends up manifesting itself systemically. It’s impossible to fight against the passage of time, but it is possible to get old with a more than optimal state of health. Take care of yourself, and your biological age will distance itself (below) from the chronological one.