Dr Kim has been neutering kittens as young as 6 weeks old for the Cat Protection Society and shelters since last century.  Kittens are very robust and bounce back quickly from their operation, but they need to be with their mother and litter mates to prevent problems.  Neutering at 4 months old is her standard for your cat (males and females), as well-fed females will reach puberty and get pregnant at 4 1/2 months old.  There is no problem with neutering at this age, and it gets the surgery out of the way so there are no ‘mistakes’ (spraying or pregnancy).  Check here for campaigns encouraging Early Age Neuteringhttps://www.youtube.com/watch?v=wWRKVP_8Hl8 . If your vet won’t do it, Dr Kim will, and she has special packages that will save you even more.