Catnip is not harmful to cats or kittens. If consumed in very large quantities, it can trigger diarrhea or vomiting, but in general it is non-toxic, not addictive and not harmful at all to cats of any age.
Catnip contains a chemical called nepetalactone. Cats have a receptor for this chemical in the roof of the mouth, in a special organ called the vomeronasal organ. Cats vary in their response to catnip. Young kittens and older cats often do not respond to it at all. Around 10 to 30 percent of adult cats do not respond to it either. Some cats become very active after exposure to catnip, while others become very calm and sedate.