I started at 19 with 1ad.
I was already 6'3 and I've been shaving daily since I was 13 so I'm plenty phisically mature.
Besides I don't wanna get any taller
I'd say the Ideal age would be 23 with 5+ years of serious bodybuilding behind you with your diet and training down pat.
I think the reason many of us started early(under 21) is
1. impatience, wanting to be bigger and leaner sooner.
2. The impending ban over the last few years has scared people causing them to rather do a cycle sooner rather than wait and never have the chance.(for those of us conserned with legal issues)