So a teenager that isn’t old enough to vote but is mentally intelligent enough to operate by themselves in society (so like 15 or so) should have more rights than they do?

If we’re going purely based on intelligence then your argument doesn’t hold water, as intelligent people who are too young to be full citizens capable of voting, military service, owning property, etc. Are still dependent on their guardians.

How would this be any different? Intelligent handicapped people exist and don’t get every opportunity that others do.