I don't think any "power-level" can really be used to determine if someone is a hero or a superhero. Say, Superman and Mr. Incredible are both superheroes, but the former is clearly a lot stronger than the latter (in most comic versions, at least). Likewise, there are stories where even the weakest of cannon fodder could easily beat Superman, so he wouldn't be considered very "super" there.
What makes one "super" is rather arbirtrary, imo, so people should just use it in any way they want to. Although if it's just being beyond what a human could achieve, then for D&D it'd probably be beyond level 20, since a "regular" person could theoretically reach that level if they work hard/are lucky enough.