I've had this test done at all ages and the vets never said "dont do at certain ages", they all said TEST them. Cindy was tested at what we think was at 6 months old approx age, we werent sure when exactly she was born. Stephen was tested at 7-8 months old (paperwork isnt infront of me at moment, going by memory). Skinny was tested at approx 1 year old.
Thumper was tested at 2 years old.
When Butterscotch was diagnoised with Feline Aids we had everyone (Thumper and Skinny) tested again.


I think it's a matter of how knowledgeable the vet is about things the younger vets seems to be more "up to date" on stuff than older vets (my experience), but when it comes to testing for stuff they are all in agreement per my experience.... Test them regardless of age.

My advise is to test at any age is better than NOT testing them at all, just seems like good common sense to me.