I had to have Scrappy Two spayed early as she was 6 moths old and was carrying a Kitten Scrappy Three who was stillborn
I have found that you should not wait too long , as if the Male Cats mature before neutering they will spray , Tubster Panther and Miquelito still spray, thankfully the scent is weak!!!
I think that 6 moths is a good time, I will ask my Vet for a professional opinion today.