I am giving you the link for the list of minimum age for marriage prevailing in different countries. Medically, suppose a couple marries early, say, at the age of 12, boy may be required to be a little older than the girl, since boys are thought to reach full puberty a little later than girls. But according to any authority, if a couple want to get married at around 18, the boy need not necessarily be older than the girl.

Now see yourself how several countries discriminate men against women (although several other countries don't do this), by going through the following link (from Wikipedia) :
http://en.wikipedia.org/wiki/Marriageable_age