Cats can safely have catnip starting around 3 to 6 months of age, which corresponds with their sexual maturity and the development of brain receptors necessary to respond to catnip's active compound, nepetalactone. Most cats begin showing sensitivity to catnip after about six months, though some may respond as early as three months. Kittens under three months generally do not respond to catnip because their receptors are not yet developed, and catnip has no effect on them at that age. Catnip is safe for cats of all ages, but it is best introduced in moderation and after they are old enough to respond to it. Around one-third of cats may never react to catnip due to genetics, regardless of age. In summary:

• Catnip is safe for all ages but typically effective from 3–6 months onward.
• Sensitivity develops around sexual maturity at about 6 months.
• Kittens under 3 months usually show no response.
• Introduce catnip gradually and observe the cat's reaction.
• About 25–50% of cats may not respond at all due to genetics.

These points reflect the consensus from veterinary advice and pet care resources.