we break them the same as the rest... about 31/2 so they are ready to show as a 4yrs old. if anything it depends on the stallion with the hormones situation. each stallion is different. one stallion we had was very docile as a 4yr old and wasnt that boysterous. and another was very bolshy and needed alot of handling, we broke him the same time as the other one though. its then depends on the way you ride them. you cant be soft with a bolshy sec D stallion under saddle, and you cant let them do what they want willy nilly or they WILL take control. xx so my answer is 3 and a half to 4 yrs old.. and, hormones in my opinion do not get in the way.
I used to compete a Section D stallion at a good level show jumping - he was gorgeous! Have worked with lots of stallions, and I treat them (as far as possible) the same as any other horse. You need to make sure that your groundwork is really good, that your horse respects you and listens when you ask something of him, and is willing to respond. The backing process should then be relatively easy. Personally I prefer to go through the backing process over a period of years, right from a foal, because to me everything that you do with your horse (leading him, tying him up, putting a rug / roller on, introducing him to traffic, leading him round the fields / woods, training him for the farrier, etc, is all part of the backing process -having a rider on is just another part of the whole thing.