* Brat: This is the most common and general term for "brother". It can be used for both younger and older brothers.
* Starszy brat: This means "older brother".
* Młodszy brat: This means "younger brother".
So, if you just want to say "brother", you can use "brat". If you want to specify whether he's older or younger, use "starszy brat" or "młodszy brat" respectively.