At that age, you should be able to tell just by looking at your iguana. Some things to look for:
Males have large jowls, which bulge out. That's where those big round scales are right behind their jaws. Males also have two fatty deposits that look like bumps on top of their head, just behind their parietal eye. They also have fairly large femoral pores - the ones on the under side of their thighs. If you don't see any of these things, you've got a female.
If you can't figure it out by looking, post a picture on here that shows the head pretty well, and we can probably tell you.