I'd say, as with any other 'hobby', maybe 20% will continue at the same level. I think this is true of any pursuit that youngsters take on, because they have so much spare time, but then the amount of spare time steadily decreases (until you retire, I guess!?).
For example, I went to school with 2 England under 18 rugby players. Neither plays rugby any more, although one played into his 20's. I know another guy who played pool internationally at age 16, and now is fat and lazy and useless at most things, although he can still kick my ass at pool!
Most people who continue do so not necessarily because they're the best, but because their motivation for doing something comes from the pure love of doing the activity itself, rather than the results. For example, I ran at school. I was pretty good, and qualified for the national championships. I don't run any more, but a friend I ran with, who was barely a reserve runner for our team, still runs. He runs because he loves running - he doesn't expect to win races. If you run to win races, NOT winning races is more likely to upset you than if you run purely for the joy of running. Same goes for cubing - if you cube to be sub-xx and DON'T get sub-xx, I guess you're more likely to give up than those who cube for the love of cubing.
The other 80% will inevitably lose interest as they get into things like the opposite sex/same sex/drink/drugs/family life/employment. Of this 80%, there will be some, sure, who will still occasionally cube, but not with the regularity or commitment that they had in their youth. There will be others who will pick up a cube at a party one day and solve it, dazzling the crowds, and remember how much fun cubing is. Maybe they'll start cubing again, and appreciate the joy of cubing rather than the agony of failing to become a world-beating cuber, and this time stick at it?!
So IMHO, those who love cubing, not love being fast, are more likely to stick at it. The rest will sell all but 1 of their cubes, which will end up gathering dust in a cardboard box, under the bed they used to sleep in at their parents' house, alongside the football boots, tennis racquet, and Twilight posters.
Edit: just FYI, 80/20 is just an arbitrary figure I plucked out of thin air based on personal experience. I'd say less than 10% of my friends still actively engage in hobbies they enjoyed in their teens.