Is the child born to a couple the 'product of their efforts'? If so, is that child always 100% owned by those parents?
No. The parents cannot “own” their child any more than they can “own” anyone else. As legal guardians, they are responsible for holding their child’s Rights in trust until they are old enough to exercise those Rights themselves.
Full neurological development is generally achieved around 18 years of age. This makes it not only congruent with the generally accepted age of majority, but backs up that age with a bit of biological science.