Every cat is a little different. Some kittens don't like catnip until they're older, others start playing with it when they're around 3 months.
I know that catnip is safe for kittens but I don't give it to my kitten because I don't want to expose him to it just yet. I only use catnip to help my cats relax when they're home alone, when we have to travel, or see the vet.
Also, catnip doesn't make all cats relax. Some cats become aggressive and overactive with catnip. Kittens are already very active so I don't think they need additional stimulation.