Actually just because you are 16 doesn't mean you wear a certain size of underwear, it depends how tall you are and what your waist size is.
What Size Underwear Does Justin Bieber Wear?
If your 16 you should know
How is someone supposed to know what size underwear he wears!? You would have to be stalking him to know what it is! Or, you would have to know him really, really well. (like his family & friends)