Did your book also mention that if the kitten hasn't inherited the appropriate genes to be a catnip responder, it won't matter how old they get, they won't like/respond to it?
A kitten with the appropriate genes to be a catnip responder also has to be at the age of sexual maturity - this varies a fair bit! I've seen in my breed, kittens as young as 4-5 months begin showing sexual maturity, domestic cats with siamese behind them (as I recall) may mature at a younger age.
I'd say if your kitten isn't responding to catnip by the age of 7-8 months, they aren't going to.
In which case, you can try the two alternatives to catnip, which some non-catnip responders will respond to - honeysuckle or valerian.