Of course children are different in the same way that adults with learning difficults are different in that they may not have the developed mental capacity to choose a partner or understand the implications of their actions.

lolwut?????????

You couldn't have chosen a worse comparison.

Adults with learning disabilities have a more rigid and less capable of introducing new information mind, even compared to other adults. Children's minds are more flexible and more capable of introducing new information.

When I say "measurable developed mental capacity in relationships" I mean that they should have a knowledge of relationships, what it means to be in a relationship and all the mental and physical aspects of a relationship.

All of these are cultural.

If these are tested, then the test is about anthropology, not about relationships.



And overall your proposal suffers the same defects that all "mental capacity" tests have. They are not neutral. They are arbitrary. They are designed by elites to have a pseudo-scientific argument to keep underclasses as underclasses. You have explicitly stated you do not want to discriminate by gender, age or race, but that is exactly what will happen with mental capacity tests.

What will happen is that predefined sets of people will have it easier and others will have it harder to pass any such tests, and instead of reducing discrimination, it will increase. Imperfect as it is, a straight AOC is superior.