If you don't know the age of the rats is there a size/weight that you go by for when it's okay to breed your females for the first time?

I have a few that I don't know the age on, but I would say they are mediums... Didn't know if that was a good starting point or not. I've heard 4 months of age, and I'm assuming they must be close to that.

This is my first try at rat breeding, so want to make sure I don't put them with the male too soon. Thanks!