I breed for show and pet in equal measure. I don’t think there’s any benefit to rats if we breed for anything less than the full package, and so I think every mating should be taking into account temperament, health, type, and the paint job (deliberately listed in that order).
It is hugely unusual to see anyone reputable offering kittens for homes that are only a couple of weeks old. Breeders should be interested in selecting which kittens they keep back for future breeding on the basis of best possible temperament, type, and colour/markings.
When you are looking at kittens that are very young, you know what variety they have and what markings, but you cannot properly evaluate temperament or type. So that means you are only selecting on what variety they are – not even the quality of it, since the colour and markings can change rapidly in the next few weeks.
If you do have an interest in breeding to improve temperament/type/colour, you should make decisions on the kittens you are keeping and those you are not as late as possible. This would allow you more time to properly evaluate the kittens, and it’s also better for those purchasing kittens, as you can tell them more about the personality of the rats involved rather than it just being about what they look like.
By properly selecting the rats you keep as potential breeders in the first place, you’ll be able to breed to improve, rather than breed just to create more kittens of a particular variety.
You will often see written that there is an upper age limit in breeding a doe, but there are certainly no increased risks of breeding a rat under a year old – and realistically if the rat is in peak health and fitness at any age, a litter isn’t going to be an increased risk no matter what the calendar says (and if she wasn’t in peak condition why are you breeding from her anyway?).
Your main issue with breeding older does tends to be that if the line has been bred from at the lower end of the breeding window for multiple generations, you may have accidentally selected for early onset infertility, and so find it difficult to get older does pregnant. I always select for later fertility as it makes my life easier as a breeder.