Max age is hard to say. Minimum age is likely approximately 2.5-3 years-- my opinion. I sometimes see people make the mistake of answering rotkeil growth rate questions based on their experience with other types of severums. I think, 'wow, mine never grew that fast' (and I've had a lot of them), then I see theirs are not rotkeils.
With the same care and feeding, Rotkeils grow somewhat slower than green or gold sevs; at least based on my own experience with each of these types and also what I've typically read from other rotkeil owners. I've spent time looking to find someone who says different (and who has also kept the other types so that they can compare them) but I haven't seen it yet.