Well, for one thing, the teen O5 didn’t replace their older counterparts, nor were they turned into murderous villains to expedite the switch. So right there, conditions were better. Additionally, those X-Men started their publication life as heroes of that age, whereas Iron Man had never been a teen hero before–so the O5 was a bit of a nostalgic return to something, while Teen Tony was more trying to force a concept into an unfamiliar shape.