Don't exactly know the 'norm' since I haven't been breeding for long enough, but my homers have varied quite a bit - some beginning as early as 4 or 5 months. Not good to let them raise babies so young, though.
We just had two pair mate up at 5 mths old one pair has fertile eggs and one seems to have laid duds. (unfertilized) I'd forgotten they could mate so young but back when we had rollers 5 to 6 months was the norm and yes they could produce healthy young but in cooler times they did not do as well. When chicks are old enough to keep themselves warm the younger birds won't always sit with them. In colder times this makes for chicks which don't survive if not sat on at night. I started seperating males from females when a chick died that way. It was horrible to see. So in the fall once the days start to grow shorter (depending upon where you live) and the coop is no longer a comfortable temp during the day it is best to sepperate as the nights can be far too cold for chicks to make it. Unless you have a insulated heated loft. Others might know tempature ranges for chick survival.