Horses are frequently done at 6 months, unless there is something really different with donkeys I would expect it to be the same. It's a good idea to get it done before they start to realize they are stallions, so you don't end up with a gelding that still wants to act like a stallion, that can be obnoxious.
You can geld as soon as both testicles are descended. With very young animals, the testicles may "come and go" at times, so you can only feel one, other days they are both down. If you only find one ALL the time, you may have to wait a bit to see if the other will come down with some age.
I still would want the gelding done before a year, so consult a Vet if both testicles are not down by then. Retained testicle can involve a more difficult gelding process, but retained is usually is not a common problem.
I second getting him gelded ASAP to retain the nicer dispostion of a gelding, never will act like a Jack because he didn't have the hormones going.
I would assume he's done like a horse or pony and 6 months is a good age before he gets too full of himself. If you have females around the boys can breed pretty young. We did have to wait til a year to do our pony colt tho because one testicle took that long to come down.