Strictly speaking, we have that already — I forget the English term but a court can declare you to be childlike and appoint a guardian to take care of adult affairs for you.

But that's not what you meant, right? You meant: what benefits would we get from having more complex "age" machinery in every database? Well, the complexity can be an advantage in itself. Much employment for highly paid database specialists, and many (small?) opportunities for blackhat mischief by leveraging people's assumptions that one "age" is the other "age".