Sport fans act in every single way exactly like anime, video-game, sci-fi etc. fans
But those sports fans wear the jerseys of other adults, not of teenagers (or worse, pre-pubescents).
It's the same reason why a man cosplaying Worf isn't weird, but a man cosplaying as his son Alexander Rozhenko is creepy.
Or the 10 year old.
The teenage boy catching big-eyed cute pseudo-monsters in an all-primary-colors world
When I was young, I hated broccoli and squash. As I matured, I started liking them. Why? My tastes literally changed.
The difference in setting, tone and style are fluff, a matter of taste and preference, and little more.
Adults playing a game with a 10 year old protagonist is a creepy as old men staring at little girls.
This is completely different from both adults and children reading the Chronicles Of Narnia and both enjoying it (though for completely different reasons: the adult seeing nuances and subtexts lost on a child).