Ways to know you have reached a certain age:

Before you do anything, you have to do something else, usually piss.

Something that lasts a long time (say, a blanket) wears out, and you
buy a new one, perhaps for the last time.

The hair on top of your head, if any, is too sparse to hold back the
hair in front, if any.

The president of the United States is younger than you are.

You get an extra meal when you floss your teeth.

You see an obituary and say "Was that old bastard still alive?"