Forgive me if someone else has already said this.
I think the best play is between 15 and 22. That's First Class.
Because that's when players find it easiest to *believe* in the reality of the game world. They believe in the reality of their characters. This is an illusion that becomes harder as we get older.
We also have ridiculous levels of free time and few commitments or distractions. So people can play as and when they want.
This is not my point. My point is what kind of play those realities lead to. They lead to the *possibility* of a lot of personal attention from the GM. Players with characters who have ambition, who want to achieve things in the setting beyond leveling up.
That leads to a highly dynamic and reasonably plausible game world, with lots of different players, all playing together but also alone, talking to the GM about the game after school, solo adventures, adventures for smaller parts of the group. Completely Ad Hoc, driven by player motivation rather than a schedule.
That's ideal. I think the stereotype of 1 GM and 5 or 6 player meeting regularly is one we inherited from the mid 1980s once D&D had become a phenomenon, but I don't think that kind of rigid play was the norm back then. It BECAME the norm because the Network of players aged and collapsed. When you're 30, you have to schedule the game and can't play in so many different groups in so many different combinations that you don't even think of them AS different groups.
So if that's ideal, and I propose it is, Online Play is nearly as ideal. Second class, perhaps, but better than the "6 people sitting around a table every week" model. Because Online Play lets you do exactly this. It lets players from all over the world play when they want, based on their own motivation.
Between all the different ways of communicating online, Email, Chat, Facebook, Skype, and then actual VTT solutions for encounters, you can roleplay when ever you want. At work, At home. Ad hoc. Based, not on a schedule, but on motivation.