At or around 6 months old is pretty standard BUT kittens can be safely done when they are much younger than that; it's a smaller operation which they recover faster from, but not many UK vets do it yet. I'll be waiting till my girls are slightly over 6 months as they didn't have the smoothest start in life, one's quite a bit on the small size for her age, the other's got dodgy hips. Also I want to make sure I'll be around and not busy to nurse them afterwards - have a feeling it'll be quite a job to stop Letty (the naughty one) from getting at her scar... or her sister's!