Its all about understanding. In my opinion, the guy shoule be around 3 years older than the girl. We'll get the reason if we just think properly..
A girl's regular age of marriage is 24 (on average) and the guyz' age is around 28; even if not 28, not atleast 24. 24 is a the age a girl will get married probably. but for the guy, 24 is too early. so i think there should be a gap of around 3 years. but again its all about understanding..and if the families dont have any problem about same age or even the guy yonger than the girl, then why should we bother about that?