The mental capacity of a person to make reasonable contracts, is the only criterion, by which to determine his legal capacity to make obligatory contracts. And his mental capacity to make reasonable contracts is certainly not to be determined by the fact that he is, or is not, twenty-one years of age.
Allah says in the Qur'an not to despise one another. So the criterion in Islam is not color or social status. It's who is most righteous. If I go to a mosque - and I'm a basketball player with money and prestige - if I go to a mosque and see an imam, I feel inferior. He's better than me. It's about knowledge.