at what age are the FelV and FIV tests accurate?

It totally depends on the situation. An exclusive relationship between
mother and kittens I would tend to agree with you, but if there is any
mingling of families, it happens. (aka a shelter environment or a large
feral colony)

On testing kittens and having experamented with tests I have never yet
found a kitten less then 12 weeks to be positive where the kittens
mother has been negative and I've also never seen a positive mother who
didn't also have positive kittens. It might be a bit of a waste of
money having to test all the kittens and put them through that where its
possible to test the mother.