Up to you what kind of story you want to tell. Just understand that it's going to be different, depending on whether she was or was not legally adult according to her society at the time she took up the shield. If the age of majority really is 21 then they could argue that the shield appeared to Ursula and therefore trumps human law but that's something that not everyone will agree about. It's going to have a big impact either way.

Also bear in mind that the farther back in time you go, the earlier "adulthood" was. A lot of cultures set it at puberty, so 12-13 was common. Some things, like knighthood or ruling without a regent, did have a later age but not always.