what is the correct age for marriage according to law?

hii sir/mam .. I'm preethi lives in Coimbatore..iam loving one boy and he is also Coimbatore.we both were loving for past 2 years.both family knows this.his family accepted my love and started supporting us.but my family didn't accept it.. my family forcing me to leave him. but I don't have idea to leave him..i need to marry him only ..and I need to know that.. what is the correct age for a girl and boy for legal marriage? some says that boy should finish 21 years of his age; that is his age should be 22 and the girl should finish 18 years of his age that is her age should be 19!! and some says that the boys age should start 21 and the girls age should finish 18 yrs!! I'm still confused about this..please conform me the correct age of the boy and a girl to be legally married under law rules..