At what age to rats stop growing?

A friend gave me his rats a month ago because he was being stationed in Japan and couldn't bring them along, and I never even thought to ask their age. I know he didn't have them very long either before he had to leave. But they've definitely gotten bigger since I've had them. One of them was so teeny I thought she might be a baby, and now she's pretty big, and the other was fair-sized to begin with and now she's huge. Now I really want to know how old they are. What does /an/ think?

Pic related, it's the big'un crawling on me before bedtime.