If you're looking at a 'good' age, from what I've read the better breeders like to wait about a year so that they're sure of the bucks temperament and any health problems. Since they aren't put under the same physical stress that a female would be when reproducing, they tend to stay fertile into old age.
But seeing as it's not unheard of for someone to buy a young pregnant female rat who's mated with her litter-mate, I'd assume that male rats become fertile at around the same time females do, so 5-7 weeks.
Why do you need to know? xD