My cousin is 14 and can do 21 pull ups and asked me if that's good. I don't do pull ups so I don't really know. Is it good?

It depends on his/her weight, height, etc. But I only do 10 pull-ups a day so it is fine.

I don't care about any of those questions^, 21 pull up's is very good.

he has good strength for his weight.

Depends on his weight, height and what not.

Nevertheless, 21 pull ups is great!

it depends on how much training he has put into the amount of pullups he could do