Indeed, age restriction on carry are very rare. Besides the ones mentioned above, Arizona has one of a sorts, in that if you are under 21 you can only carry a common pocket knife, though overall AZ's carry laws are some of the most relaxed in the entire US to the point a GUN can be carried without permit when over 21. It's a very carry-friendly state so even a minor with a bigger knife would have to be doing something far more dangerous than just carry alone for cops to give them a second look.
It should also be mentioned that actual laws on sale to minors are very rare too. Stores just refuse to sell to minors as a personal choice because if the minor cuts themselves, the parents will no doubt sue the store. Stores love to lie about this and say "sorry, state law" but that is just to keep customers from whining and arguing with them.