 You know the answer to that question, but the reason you know is because you were told by your parents and because you have a birth certificate. If you didn't have that, it could be very difficult to estimate your own age, let alone the age of something else or something like the age of the Earth. So those are things that you have to keep in mind when we're talking about this issue, about these clocks that are used to try to date the age of things and the age of the Earth itself.