I think it depends on how well developed you are. A fair few start at around 18, whilst others are in their early 20's. Matt Skelton was 35!
It's not just baout your body but how mature you are mentally. A big amateur carear can really help the 2nd one. So it really depends. Although turning pro early is never a problem as long as you take it slowly with the matchmaking. You can learn through pro experience, gym experience as well as an amateur carear. There really is no perfect age, it really depends on a lot of factors.