Question: how does the blood help to know how old you are

Anastasia Aliferi answered on 11 Nov 2019:

Our blood (as almost everything in our body) contains our DNA, which is basically a code that tells our body how to work. I’m actually looking at that DNA.
Now to the tricky bit, our DNA doesn’t change as we get older – or we might turn into bananas, who knows?! But even though we always have the same DNA we get gray hair and wrinkles, don’t we? What we’ve found is that there is a substance, much like chewing gum, that sticks on that DNA and the more it sticks the more difficult it is for it to work. For example the part that makes our hair the correct colour gets so much gum on it that in the end the code is hidden and our body has no option but to give it no-colour, which is white!
So what I’m doing is looking at the amount of gum on our DNA and from it I can calculate how old someone is.