Have you ever watched little kids meet at a park? What's the first question they ask each other?
"How old are you?"
The reason why children ask this is because they identify with kids their own age, and more easily become their friends. Kids the same age are considered "us". Kids who are older or younger are "them."
Most adults no longer make this alienating distinction, and accept people of all ages as friends.
When does one make the transition from child to adult? It's often during college ages. Since everyone matures at different rates, this transition may happen sooner, or later.