At about 4months ish. It depends on the vet, the buns need time to sexually mature (their balls need to drop!) also they need to reach adult ish size so they cope with the anasthetic as well as they possibly can. Different vets make different judgement calls on this so I can't really be more specific.
If your bucks are spraying and being very smelly I would imagine that they have either reached sexual maturity or are getting close (I am guessing tho here!)