The problem is that this will pigeon-hole kids into a sport they're prospected in at far too early an age, meaning they'll focus on it year-round and not pursue other sports or interests and therefore stunt their physical development (not to mention their mental/emotional development). You know that as soon as some (most?) parents get wind of little Johnny or Jenny having the attention of recruiters, it'll be three seasons of one sport rather than three sports each year, plus camps, trainers, coaches, etc.
Gymnasts (and most Olympians) don't turn their amateur careers into professional ones, so there's no real equation in circumstance. Plus, gymnastics by its nature is a much more well-rounded physical endeavor (abusive coaching practices notwithstanding); however, the competitive age is far too young IMHO. You can't constantly expose developing bodies to those kinds of forces without some long-term health consequences. Just because it's legal doesn't mean it's right.
The Mary Jane comparison doesn't hold because, much more often than not, kids are pushed into these situations by coaches and parents--pot doesn't usually come from mom and dad. Usually
Athletics makes money. Smoking up generally doesn't.