Hi Jill-
I'm wondering what breed of chickens you have....I think some breeds mature faster than others. Silkies, for example, are very hard to sex....at least they are for me! Now, that agressive behavior you're seeing could be male or female. There are hens that, when they're at the top of the pecking order, will be agressive like a rooster. Some of the things to look for on a young chicken that may indicate it's a male are those longer tail feathers, and longer hackle feathers (neck feathers), bigger combs and wattles (or is it waddles), and of course crowing. Also, a rooster will have a heavier body and thicker looking legs too sometimes.
Hope this helps! Lots of people put pics up if they're not sure, and then members help guess......