Question: how do we know when someone was alive or how old something is without any records

Adam Ruben answered on 9 Mar 2018:

There are a lot of ways of figuring out the age of something. One is called carbon dating. Living things are made of many elements, but mostly carbon, nitrogen, oxygen, and hydrogen.

All of these elements have neutrons (except hydrogen), but the number of neutrons in an element isn’t a set rule. For example, carbon should have 12 neutrons, but sometimes it has 14. Let’s call the two forms C12 and C14.

C14 can be taken up by animals when they’re living, but not when they’re dead. (They eat plants with C14 in them, but when they die, they no longer eat anything.) So when an animal dies, it has a certain amount of C14–but then that C14 starts to leave the dead animal over time. We know how fast C14 leaves the animal’s body (it becomes ordinary C12), so based on how much is left in the body, we can figure out how long the animal has been dead.

That’s just one way of measuring this. There are several others that work on similar principles.