You may be right (and probably are especially since it would allow greedy governments to confiscate wealth), but I just thought that to be legally considered gambling, there had to be some money at risk by the participant.
I’m just guessing here, but I think the bigger reason for excluding minors is that ESPN likely requires winners of contests such as these to allow their names, likenesses, etc. to be used by ESPN for promotions. That might be problematic for minors since such contracts are likely not binding upon minors. Like I said, though, that’s just a guess.
As so often happens in law definitions are fluid. Especially when crossing the minor line, what states consider gambling for minors they might not consider gambling for adults.
Image and likeness probably plays into it too. Minors preset a large and convoluted can of legals worms best left unopened.