Is it illegal for a citizen of India to come to America on a marriage visa with the intention of marrying a U.S. citizen if he is under 21 years of age?
No, not as long as he's 18
The states determine legal age of consent of marriage. Not US law
Yes, a marriage performed in South America will be honored in the U.S., with the exception of gay marriage- which is not recognized in the U.S. If you mean a marriage in the U.S. between a citizen and an illegal, this marriage will also be recognized, but again, not for gay marriage. Be aware than simply marrying a citizen doesnt grant you legal residency