At What Age....

I have left females in with their father's colony, so they probably became pregnant the very first time they came in heat. I have seen no negative effects from this. It is possible that their first litters are pretty small, although in a colony situation it is often hard to know how many pinks came from which mom. Also, in this situation, I don't necessarily know how they would do in terms of a long productive life, because the other mice in that colony are already at least 2 litters ahead of them, so when I retire the colony, these females have had a shorter breeding career.

It is much more typical for me to wean the babies at roughly 3 weeks, raise them up for another week or 2, or possibly longer depending on my needs at the moment, and then put them into colonies. The females that are put in colonies at 4 or 5 weeks of age do tend to have slightly small litters for their first litter, but they care for the babies just fine and don't appear to suffer any ill effects. They go on to have long and productive breeding careers without a break until I retire them.

I don't retire my breeders at a certain age, either. I keep records of how many babies I wean from each colony (so any that don't make it that far don't get counted), and when those numbers drop off for 2 cycles in a row, I retire the colony. I have noticed on occasion that a colony will have one cycle with really low numbers, then bounce back and have high numbers again. Based on this observation, I believe that nature provides a way for the females to have a "break" if their bodies need it, by having them produce an exceptionally small litter.

Mice wouldn't be able to breed so young, and cycle so frequently, and be able to get pregnant again so soon after giving birth, if they weren't designed to produce lots of babies over their lifetime. If YOUR mice can't handle it, than I would think it is more likely a problem with either the genetics of your colony, or the diet you are providing, rather than that mice as a species can't handle it.

What does everybody mean by "retire them"

I don't even pay attention to age. I just leave 1 male and 3 females in the same tub. Eventually they will get pregnant and have their litters. I just had a female that was only 5 weeks old get pregnant and give birth to a litter of 9babies. I know she was only 5 weeks old because I purchased her when she was 3 weeks old (right after weaning) as food, but I ended up keeping her because she started to get huge out of nowhere.... lol

You can follow strict guidelines all you want but your basically just making more work for yourself. Male+Female+same cage=babies. It's simple.