-A curious adult from California February 25, 2015 Until recently, scientists could not look at someone's DNA and figure out how old they were. But that all changed in 2013. It was then that scientists noticed that as people age, they build up the same set of specific DNA changes at the same times. So, 20-year-olds might get a certain change, 30-year-olds might have a different one and so on. Scientists can then look and see what changes you have. Oh, he has a set typical of a 53-year-old and she has a set typical of a 29-year-old. And so far, it has really worked! This is a different way than scientists might have thought a few years ago. Back then scientists thought they might be able to tell someone’s age from how long his or her DNA was. Each time one of our cells divide, the DNA in the cell gets shorter because a little bit of DNA is lost from each end. To keep important bits of DNA from disappearing, we have long stretches of DNA at each end called telomeres. Scientists used to think if they could measure the length of a person’s telomere, they would be able to tell their age. After all, the more times a cell divides, the shorter its DNA will be. And the older you are, the more times your cells will have divided. Unfortunately, it wasn’t so simple. It turns out that different people start out with telomeres of different lengths. You need to know how long they were to start with to figure out how old someone is. Most of us don’t have that information. We now know it is much easier to tell a person’s age by looking at specific DNA changes that happen around the same time in most everyone. For the rest of the answer I’d like to focus both on what these DNA changes are and why they tend to happen at the same time to different people.

Genes Can be On or Off Our DNA has the instructions to make us who we are. These instructions come in the form of genes. Each gene has the instructions for one small part of us. Every cell has the same full set of instructions for building and running a person. But of course, not every cell looks and acts the same. A nerve cell is very different from a muscle cell! A nerve cell looks different because it only has the instructions (genes) turned on that it needs to be a nerve cell. The ones specific for a muscle or a liver or any other cell are shut off. The same is true for each cell type. Different cell types are not the only reason a gene may be on or off. Sometimes as you get older, you don’t need cells to do certain jobs anymore or they need new jobs. For example, as we develop in the womb, all of our cells are growing and dividing like mad. But once we are an adult, these cells don’t need to do this anymore. In fact, if they do you can end up with cancer! Cells control which genes are on and off in lots of ways. For this answer, we will focus on something called epigenetic marks. Marking up the DNA As mentioned above, epigenetic marks are one way the body turns on or off a gene. One way that cells can mark up their DNA is by adding small chemical groups to certain parts of it. These chemical changes act like a switch that tells your body to turn a gene or set of genes on or off. So, if your body doesn’t need the gene to make liver cells anymore, one way it can turn it off is by adding these chemical groups to the DNA. These epigenetic marks are also important because they are what help to make each of us different from one other. Each of us has our own set of epigenetic marks that tell us, in part, what to look like and who to be. They are what make each of us special! It turns out that this is a big reason identical twins aren’t really completely identical. Even though they have the same DNA, they are different because they each use it slightly differently. They have a slightly different set of epigenetic marks. But identical twins and everyone else do share some of these epigenetic marks in common. And some of them can be used to tell how old someone is. The Marks of Aging