Boy has 18 years,girl has 24 years is this marrige legal in india?

In india the marriageable age of a boy is 21. But already married means, the court can only declare it as null and void.After that only you can proceed, after attaining the age of 21, for legal marriage.I cannot understand, I wonder what made this boy to get married at the age of 18 that too with a girl of age 24?