My friend gelded a 6 year old, it took a while for the stallion tendancies to subside and he was always bigger and bolder than the other geldings (and a bit of a bully!) but within a year or so he was able to go out with mares without harassing them!
Clearly it's not ideal but in the UK stallions are very difficult to keep at livery given their special turn out needs and the fact many people are wary of handling them
We gelded a 10 year old once when we could no longer use him as we kept most of his daughters and we wanted to keep them and him as ridding. gelded him on the saturday, and he was fine a day later, proberly didnt even realise what had happened, by about a month later he was stabled and lived out with our other geldings. put on alot of weight though since he was gelded.
Shouldn't be a problem. My colleague gelded a 15 year old the other day.
There is a theoretical slightly higher risk of complications, and you will probably find that post op swelling is more severe as the balls are bigger so there is a bigger hole to fill!
Do bear in mind that behaviourally he may retain some stallion characteristics, especially if he has been used as a working stallion, they are very variable about this. Some will behave almost as if they have never been gelded, others will be like they never had testicles! And everything in between.