I had this discussion with my Vet when it came
to neutering Rocky.
My vet also does not recommend spay/neuter till
the age of 6 months of age (before 1st heat.)
He feels their is not enough long term evidence
on the effects of early spay/neutering.

He does make an exception to early spay/neuter
for shelter animals before they are adopted out.

Rocky is 4 years old now, so maybe his opinion has changed.