Catnip sensitivity is hereditary in only about 50 percent of cats and doesn’t develop until a cat is about three to six months old. Senior cats seem to lose their sensitivity. Average-aged cats with the catnip sensitivity gene, get a strong instant high by inhaling nepetalactone, the organic compound found in catnip.
Nepetalactone binds to receptors inside a cat’s nose, which stimulates sensory neurons leading into the brain triggering intense, almost intoxicated-like reactions.
Source: Happy Puppy Tips. Link.