Late in seeing this, and I apologize in advance, as I didn't read all the replies, so this may be repetitive.
In all the research I've done when researching this for a faq, what I found was that not only must a cat inherit certain genes to be a catnip responder, most cats who are a catnip responder, will not respond until the age of sexual maturity. And that can vary depending on breed, or mix of breeds in your kitten's heritage (i.e. my American Curls matured very young - even younger than some other American Curls - in part I would guess due to the siamese/oriental breed genes giving my guys their colorpoint gene...so my guys were beginning to mature at 4 months).
I'd say wait until they are about 4 months old and then use a fresh, potent bit of catnip, or very fresh toy, to assess if they respond.
What fun! For a 4 month old, a catnip ball toy would be great for those kitten-soccer games