I think the story is made for people that wish to be young / kid again. Like that one woman that enrolling in highschool bc she missed being taken care for... The same way many isekai are made to please people that want to self insert in a protag that gets a second chance, escape consequences of their poor choices irl and redo their life but with OP powers, loving and wealthy family and people that loves them.
don't lump shugo chara into this.
I agree amu has controversial scenes with ikuto who IS 17-18 but he doesn't end up with her in the anime, and though it's heavily implied that he might end up with her in the manga, he literally says that he will wait for her to grow up first. and also the manga is still on going and there are more love rivals so it might even change.
you could have picked so many anime, new AND old, but you picked the one that didn't even have confirmed age gap as the end game?? why shugo chara of all anime bruh

An adult became a 10 year old and is being romanced by an adult