Not bad idea for a story these aspects of super heroes stories should be covered in a way that makes sense. It is not like anything is being hurt if they made the super hero age 21 in marvel.They either become a vigilante which means the tropes of the genre become more important like secret identity and avoid the law even though they are doing good. OR oversight becomes more of a thing we get something like my hero academy in Marvel which isn't a bad thing and you get something like Strange Academy.

Because of how the genre works less heroes who are under 18 is better imo. Remember characters don't get aged up(see X-men) to keep older characters at their peak and younger heroes tend not to be taken seriously in big stories which hurts their ability to become the next big hero. So if there is a shift of young hero being 15 -18 to 17 -23. The young heroes are better off because 18 is a better threshold for stories and you can have hero who you can comfortably have working,having sex and being relationship so being trapped at that age works out better. It is a little more realistic and Metawise it is better for the hero stories.