riddle - Rarer for younger, yet also the opposite

I am a common word that refers to a person.
I rarely refer to very young people – in fact, the older a person is, the more likely they are to be called me.
However, if you pick two random siblings, the younger of them is more likely than the older one to be called me.

What word am I?

Answer

Along the lines of AHKieran I think the answer is actually

Aunt/Uncle

I am a common word that refers to a person.

Many people are aunts/uncles

I rarely refer to very young people – in fact, the older a person is, the more likely they are to be called me.

The older you are, the more likely it is that your siblings have children.

However, if you pick two random siblings, the younger of them is more likely than the older one to be called me.

In a random family, it is more likely that the older sibling will have children first and that the younger siblings will become aunts/uncles before the older ones do.