I think parents point was that being 18 doesn't demonstrate said capacity either. Some kids are mature enough to make important decisions at 15, some kids can't make a good decision if their life depends on it at 25, so the blanket statements one way or another are obviously bad.

We chose an age that made sense to us, because the law needs an objective anchor point, not a philosophical one. However, just because the age makes sense to us legally and socially, doesn't mean it's enshrined in natural law, and we shouldn't necessarily view it as such.