As others have said,, as soon as they are going through the motions and mounting the hens they are pretty much ready to go,,, they may not be quite as potent yet but they will be getting the job done to some extent. I have a Black Minorka who started at 9 weeks and was most definitly potent at that age (about 50% with first hatch)
Another thing is generally the lighter the breed the faster they will mature,, I have read cases (documented,, not just word of mouth on a forum) where Egyptian Fayoumis can start breeding in as little as 6 weeks,, larger breeds tend to start closer to 16-20 and cochins,, well those bad boys take their sweet time

As far as loosing it,,, it is pretty much agreed that they hit their peak their first year and it's pretty much downhill from there,, i keep my breeders for 3, but toward the end of the third year you can see significant loss in fertility rate.