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A person's chronological age doesn't tell us much about the health of their body's various systems. That's why scientists are beginning to study biological ages, and it turns out there may be a lot of them.

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How old are you?

You might think it’s a simple question,  but it turns out your chronological age and your biological age might not be the same. Confused?

Well, it’s actually  more complicated than that: different parts of your body age differently, too. If you’re 30, you might have the  immune system of a 40-year-old, but the kidneys of a 25-year-old. Or maybe you have a youthful immune system but some age-associated metabolic issues.

So you don’t have a biological age per se. More like several of them. How do we know different  parts of us age differently?

Why do they? And can we intervene  to anti-age specific organs? The science may be young, but we already  have some answers to those questions. [♪ INTRO] Thanks to Brilliant for  supporting this SciShow video!

As a SciShow viewer, you can keep building  your STEM skills with a 30 day free trial and 20% off an annual premium  subscription at If you’re 30 years old, you might assume  that all of your body is 30 years old. After all, your body all came together  30 years ago at roughly the same time.

But that doesn’t necessarily  mean you have the immune system or liver of a 30 year old. Different parts of your body  can age at different rates. And scientists refer to those  as your biological ages.

Now, you may be thinking: this is all semantics! Just because your kidneys are in  worse shape than the rest of you, that doesn’t mean they’re  literally a different age. And that’s true.

But by studying  systems in the body this way, researchers can identify risk factors  for chronic diseases more easily. So, how are biological ages determined? By measuring biomarkers that can be  collected accurately and reproducibly.

A biomarker can be something  as simple as blood pressure, or more complex like the levels of certain  proteins or hormones in a part of the body. For example, biomarkers like the  substance bilirubin in your blood could be used to predict liver aging, which  predicts the severity of fatty liver disease. This could help catch problems early,  and it’s easy to get a sample.   Scientists have been studying  biological age for awhile, but a study in 2020 really  advanced our understanding of it.

In that study, biomarker data was  collected from a group of 106 participants, ages 29 to 75 years old, through blood  samples, nasal swabs and stool samples. There are some well-established aging  biomarkers such as the length of your telomeres, the protective DNA  sections at the ends of chromosomes. But by using a group with a large age  range, this team was able to establish other age-correlated biomarkers like certain  proteins, lipids, metabolites and hormones.

At the population level, it was  easy to find the average levels of those biomarkers for a specific  organ system at a given age. The study assigned biomarkers  in four major systems: immune, metabolic, kidney and liver. Let’s take some of the age-correlated biomarkers they discovered in the  immune system as an example.

Your immune system has proteins called CCL27, ORAI1 and ITPR2, that help regulate stuff. CCL27 is involved in making sure your tissues have the right numbers of immune cells in them. ORAI1 helps coordinate the activity  of some of those immune cells.

And ITPR2 contributes to cell  signaling in immune response. They all have a role, but if you have a lot  of these immune-related proteins in your body, you’re going to have more inflammation. Increased inflammation is associated with aging in other parts of the  body.

Inflamm-aging, if you will. So the research team was able to show  that certain biomarkers are correlated with aging of particular organ systems. But the study was able to do more than that: they followed the group over  the course of four years, so biomarker levels could be studied over time.

Compiling all this data over the  years enabled researchers to develop what’s called a deep phenotype for the  individuals, an indication of the rate of aging, even showing that different organ  systems were aging at different rates. But why do some organ systems  age faster than others? Researchers aren’t sure, but it could be  because organs are made of different tissues, which in turn are made of  different kinds of cells.

Some cell types have faster turnover than others. For example, the small intestine replaces  its epithelial cells about every four days. But neuron turnover in the brain can take decades.

Each time a cell divides, it contributes to aging. But cell turnover doesn’t explain why organ  aging seems to differ between individuals. The culprits there seem likely to be the  usual suspects: your genes and your lifestyle.

You could be born predisposed to premature aging in a particular organ, just based  on small variations in your DNA. As for lifestyle, it could  be things like stress, habits or medical history that influence organ aging. More data will give us a better understanding  of what factors cause an organ’s biological and chronological ages to diverge, but it’s likely you have  some control over your ages.

One of the study's authors had some ideas: metabolic-agers could exercise more,  kidney-agers could drink more water, liver-agers could cut back on alcohol, and immune-agers could take anti-inflammatories, like turmeric. Of course, these systems are still related. Just because you have different  ages for your immune system and your liver doesn’t mean they’re  totally independent of each other.

An anti-ageing intervention for one  system can have overall benefits. In this video, we’ve mostly focused on one study, but heterogeneous aging is  an active area of research. Another team of scientists used the same  massive dataset to identify five ages: the reproductive system, sensory  systems, gastrointestinal system, nervous system, and connective tissue.

Using a larger group of  participants, yet another team added five more ages to the original four:  cardiovascular, physical fitness, sex hormones, facial skin  features, and gut microbiome. And other researchers split  the brain into three ages: gray matter, white matter, and brain connectivity, along with seven other organ systems. So how many ages do we have?

It depends. Turns out, age is just a number…  or maybe it’s 10 numbers. The point is, your age doesn’t define you.

And you have more agency to  change it than you might think. Thanks to Brilliant for  supporting this SciShow video! Brilliant is an online learning  platform with thousands of interactive lessons in science,  computer science, and math.

Brilliant’s Multivariable Functions  course can help you learn about what tools you can use to analyze complex  systems that are all around us, from the economy to the weather,  and yes, our own bodies. Some people learn about that  complicated stuff in college courses. But Brilliant makes learning  accessible to everyone.

Their courses are developed alongside experts from accredited universities like  University of Chicago and Duke. To try it yourself for free for 30  days, visit or click the link in the description down below. That link also gives you 20% off an  annual premium Brilliant subscription.

Thanks for watching! [♪ OUTRO]