April 29, 2004
The short answer is that at this time we can't exactly figure out how old someone is from their DNA. However, it is fun to think about how this might be done in the future by looking at how DNA changes with age.
As we get older, we actually lose DNA. Each time a cell divides, its chromosomes get shorter because they lose a little DNA from each end (see below for an explanation of a chromosome). To keep important bits of DNA from disappearing, we have long stretches of "junk" DNA at each end of the chromosome called telomeres. Once the telomeres run out, a cell stops dividing. Many cancers can keep dividing because they can add DNA to their telomeres.
Could we guess someone's age from how long their DNA is? Unfortunately, it isn't that simple. Everyone is born with different lengths of DNA. What this means is that without knowing how long someone's DNA was at birth, you could only very roughly guess their age. However, if you knew how long a telomere the person started out with, you could more accurately guess their age.
Another possibility is to look at how many changes or mutations someone has in their DNA. As we get older, our DNA gains more and more changes (this is why cancer risk gets higher when we get older). Since we have some idea of how fast these DNA changes happen, we might be able to guess someone's age by comparing their current DNA to their DNA when they were born.
Another thing that changes as we age is which genes are turned on and how much the genes are turned on (see below for an explanation of a gene). Scientists have figured out that different sets of genes are turned on at different stages of life (childhood, adolescence, adulthood, old age). It is possible that such information could give you a rough idea as to the age of a person. For this you would need more than just their DNA, you would need some tissue as well.
As you can see, while DNA can be used to learn all sorts of things about a person, at this time DNA isn't like tree rings or tooth enamel -- it can't tell you your age.