How old does a Molly fish have to be? ?

I have four dalmation mollys that were born on March 29 2011, two males and two females. And that makes them around 4 mo. Now what I'm wondering is what age can they give birth cause they look like they are pregnet. Is that possible? At a young age? I mean I know that it is possible because I have other males in the tank and it's bound to happen I just thought they would have to be older then that.