I wholly agree with your initial sentiment, although I envision it being structured differently. I think it makes sense for a person to have a name when they are growing, especially one given to you by your parents since they are (typically) a huge part of who you are at that point in your life.

But, no one stays who they are when they were 7, or 12, or 16. By the time they’re not a minor I would argue that they’re hardly the same person. Thus, I think it should be expected and tradition for people to change their name once they truly become individuals.

And I think it doesn’t even have to be a legal thing. Parents can just be like, “think about and pick your new name” and once the person decides, they (and everyone else they inform) just starts calling them that name.