You make an excellent point and so William Carter, son of Mary, d.1794, and John Carter d.1772, probably didn't need a guardian in 1785 if he was old enough to marry in 1773.
What does not add up for me is that William and Fleming, sons of JC, were having a guardian appointed (Isham Carter) in 1785, when their sisters, Susannah and Mary, chose their guardian (George Carrington) in 1774. I suppose there could be such a large age difference. I think you may have suggested that these were children of two different marriages, which may be why Susannah and Mary were not mentioned in Mary's will.
There are a couple of William Carters who marry in Powhatan in 1794 (Kedziah Davis) and 1798 respectively (Jane Scott), whose date of marriage make more sense, but I have to ties yet.