If I remember correctly, the NHC code of practice suggests between four and eight months old for a female's first litter, and no more than three litters in a lifetime. I think it depends on the hamster a bit too, my four month old female I would not say is ready for a litter yet, she's only just growing out of what I think of as her awkward teenage phase, but a hamster that develops quickly might be at four months old. Similarly if a female recovers quickly from a litter, three might be doable, but if she doesn't, two or even one in a lifetime might be all she has.
As for boys, this summer I'm hopefully breeding my LH rust boy who is two in August, and he is definitely still very interested in the smell of lady hams! I've read that he's likely to not give me as big a litter as he is past his prime, but he is such a gorgeous hamster I feel like I need to breed from him
(It's only the house move I'm currently stressing out about that has stopped me from breeding him already, his girlfriend is here and ready for him!)