While gaming companies are perfectly happy to have teenagers play they're game the targeted demographic for M-rated games is usually late twenties and early thirties. Why? Well, they can easily buy the game, they have a fair amount of disposable income, and they are just beginning the soul crushing part of life we call 'employment' so they're just looking for an escape. Teenagers really can't compete. They have less money depend on unreliable parents or siblings to get them games and have a lot of other things competing for their attentions.