question
I'm 27 and my brother is 24.

In this situation, if I introduce my brother what have I to say?

I have a younger brother. or,
I have a young brother.

Question

question
I'm 27 and my brother is 24.

In this situation, if I introduce my brother what have I to say?

I have a younger brother. or,
I have a young brother.

Answer

I have a younger brother.  
Or
I have a little brother.

Answer

'Younger' is more natural. You are saying that he is younger than you.
'Young' might be appropriate if he was actually young - e.g. if he was under 10 or so.

Answer

If you are introducing him to someone else:

This is my younger brother
or
I'd like you to meet my younger brother

Answer

I have a younger brother