The Bible doesn't really say. It says that he was part of the Pharisees religious group. We know that he originally persecuted followers of Christ. It is thought that he was born around 2 AD and the Martyrdom of Steven occurred around 32 AD. Shortly after the martyrdom of Steven is when Paul was changed and became a follower of Christ. So my educated guess would be that he is in his early 30s.
Hopefully this helped.