>>Well if people don't have the mental capacity or rights to give consent until they're 18 then it should probably extend to consent in general until 18.

When the AOC started to be raised in the 1880's and 1890's in England and the American states, the idea of "consent" had nothing to do it - not that that invalidates the issue of consent as being relevant to the AOC, but in order to put it into a historical context.

What makes sex a world apart from every other activity in the world? Why does this "mental capacity" BS relate to sex but (virtually) nothing else?

Larger issue - what weight does any argument have pertaining to "mental capacity" or the like if the sex itself is not causing them any harm?