Invert the premise.

Instead of petitioning for approval to be treated as a fully capable human being, try a protective system where the burden is upon those claiming that there is a need to protect a party from themself.

We already have the concept of legal conservatorship.

It wouldn't need much tweaking. It could afford protection without assuming that any healthy demographic feature automatically confers incapacity.

I like your way of trying to eliminate the last double standard.

But do beware that many folks love double standards. You might find yourself accused of absolutism and idealism for having the "silly" notion of a single set of laws, inclusive enough to accommodate all cases.

Dante