I have bred horses for a very long time and worked for really good breeders before that. At the Thoroughbred farms we always gelded 6-8 months. The Quarter Horse and Paint farms always gelded around a year. I usually geld all my colts between 8-12 months. Now, there is the colt every now and then that starts acting studly WAY early and that one will get special attention early--around 5 months... it has been known that a colt can get a mare pregnant as a yearling, though not common. It depends on your colt and, as you already said, what physical characteristics you want him to develop prior to gelding. If they have the genetics for great muscling, they will still develop it--just not to such a great extent. Good luck with your decision!