we can't give you his age, Rosamaria. His date of birth is not recorded, and as far as we know his age at any specific date is never given. The only qualification is that he is called a "young man" at the time of Stephen's stoning. However, what does "young man" mean? In Jewish tradition men were not really taken seriously until after the age of 30.....does this then mean Paul was under 30 when he held the coats at Stephen's death? Even then, we don't know when Stephen's death was.......except it was sometime after JEsus' death in or around 33AD.......also not a sure date.
We just do not know, except to say that he was no longer a young man when he died.