I agree with the fact that each case is different and should be looked at as such.
I myself took my gelding (then stallion) into the ring last year (at the NWMHC Schooling Show) for the first time. He was awful...rearing, biting, kicking, screaming. But it was experience for both of us. He has since been gelded and attended the Oregon State Fair that same year. He's heading for the Schooling Show again this year.
Along with him, I will be taking my other stallion into the ring for the first time. I have been and will be practicing with him hard beforehand. I am prepared for the worst and will handle it should it happen. He has been a very good boy here at home, but I also know how strong he is and keep a very good, close hold on him at all times. I will be using a stud chain in the ring.
I think it depends on the person and the horse. If they know the horse and are prepared, then great. I wouldn't sell a stallion to a newbie, no matter what age, and expect them to go into the ring the first year. I've had my stallion since he was a year and a half. I know his personality and his quirks. You don't learn those things overnight. They take months, sometimes years to learn and prepare for. Just my opinions...
Stallions aren't for everyone.
Erin