Nepetalactone is not addictive or harmful for cats. The behavioral response can be very dramatic in some cats - rolling, licking, rubbing, drooling, jumping, running. Other cats appear to become very sedate after exposure.
Very young and senior cats do not respond as much, or at all to catnip. Additionally, the feline response to catnip is genetic; 10-30% of the cat population does not respond to catnip at all, at any age. Some cats are genetically "programmed" to respond to catnip, some aren't.
In any case, for all of the (sometimes entertaining) behaviors seen, catnip is completely nontoxic to cats. If a large quantity of fresh catnip is consumed, you may see some vomiting or diarrhea, but this is rare and self-limiting. If your cat experiences this, limit or withhold catnip.