Should there be an age limit for catnip?

Yes, you are just a weirdo.

If you want to deprive your kittens from safe pleasures, that is your business.

Catnip is not a drug. From a veterinarian’s website:

Catnip actually belongs to the mint family. Catnip is not a drug, so you don’t have to worry about your cat getting addicted or suffering side effects. The plant contains an essential oil called epetalactone, which, as you may know, has a strong effect on kitties. Despite the advances of modern scientists, much about cats and their behavior is still unknown, but experts generally agree that this essential oil mimics Kitty’s ‘Happy Pheromones’, which would explain why your cat suddenly turns into one happy ball of fur when she smells catnip. Interestingly, cats have olfactory receptors made specifically for catnip. Mother Nature just may have made this plant for the sole purpose of making kitties act silly!

Our cat Felix just turned one year old. He showed no interest in any of the catnip toys until the last month or so.

Modern pharmaceutical drugs are chemical concoctions devised by human industrialists to mess with our biology, that pretty much all have unwanted side-effects even in their ideal forms. And they are produced by greedmonger giant corporations who hire teams of lawyers and accountants to minimize their costs (such as lawsuits) because they are driven to always try to increase their profits at practically any cost. Therefore, when they decay beyond the time they’re tested for, they become something not so tested and that mainly does side-effects or at least should not be taken expecting the same results as the fresh stuff. Also the company wants to avoid any costs due to lawsuits or other negative effects of people taking old drugs (including discouraging re-sale by other than approved people, and reducing any glut on the market so they can keep cornering the market on supply).

Meanwhile, catnip is just a plant. When it gets old, it just becomes harmless leaf residue. Also, the people who sell catnip toys or nip like long shelf-life, and aren’t concerned with the catnip black market. Also what could they do about it? GMO megaevilcorporations have not yet GMO-ified catnip and claimed patent ownership, so people can still grow it without being arrested, and dried catnip seems to remain potent a long time, in some cases even after being nommed on for months.

Yes.

There should be limits as catnip loses it’s properties as it ages.

Is there an age limit for Johnny Walker?
HELL NO!