6-8 months is when i suggest for the chis...
simple reason for waiting till older is because larger breeds take longer to mature, a chihuahuas is done growing in terms of height and lenght by 8 months, a great dane is still growing at 2 years...
by neutering early on the larger breeds it can cause growth problmes, however with toy breeds this isnt a concern.
the only hold my vet has is she preferes to not spay/neuter the dog before 1 year if its under 2lbs...