First thought I would think that its due to relationships that come with age, they say that '' the younger you are the better you play '' however I believe its false because the problem that lies with age is the relationships that are very hard to back off from. If you are young lets say 16-20 you can get away without a girlfield but when you are 26-30 women start to seduce men like crazy because they know their faces and body is changing for worse and they want children so the players who can get away with playing all day become a good target for men seeking girlfriends in turn men cant play anymore because they are busy having sex, going out and women most likely object their desire to play games and want them to start working regular job and eventually women become pregnant in the end. On the other hand, young players like 18 years old ones, play the game all day and every day in turn becoming masters and go to qualify in offline tournaments.
Secondly, young players are not busy doing anything else but play, they just live with their parents and dont pay taxes, dont work, theyr minds are not crammed with toughts about life they just play as it should be, while older individuals even 30 year olds have to pay rent and work do household chores and just cant focus on 8-10-12 hour gameplay schedule and it makes then harder to compete against young but always busy playing players who end up becoming professionals.
Considering the growing prize money payouts from tournaments I predict that in the future professional players going to be in their 40s because they will see a potential in making a lot of money from playing games and see it as a job rather than hobby where someone got lucky. Besides, entry in to video games is open for everybody the only requirement is to play all day and all the time.