Justin Bieber was only 16 years old when he first started posting videos on YouTube. He also played four different instruments and he mostly taught himself: piano, drums, trumpet and guitar. So, I imagine if Bieber had not been a superstar he would have done something in the world of music.
He may have been a music producer or a band musician. He could have been a concert or record producer or a music journalist. He could also have maybe been a music teacher. Who knows? Chances are however he would have been just what he is – a music superstar.