I just looked through Section 12020 of the Cal penal code, where a lot of the knife stuff is, and there doesn't seem to be anything about ages. It refers to things that "persons" aren't supposed to do, but doesn't refer to minors versus adults. I suspect that local laws and individual store policies are at work here. For example, here in San Francisco, the police code prohibits transfers of "weapons" to minors. Here's the SF code:
SEC. 1293. PROHIBITING THE SALE OR TRANSFER OF KNIVES TO MINORS OTHER THAN BY PARENTS OR GUARDIANS, EXCEPT FOR TABLE KNIVES AND KNIVES CURRENTLY PROHIBITED FROM SALE UNDER STATE LAW; PENALTIES FOR VIOLATION OF THIS SECTION.
(a) Definitions. Knife includes, but is not limited to: any instrument with a cuttingblade, made of steel or other similar durable cutting surface, capable of inflicting bodily harm.
(b) Ban on the Sale of Knives to Minors. Notwithstanding any provision of this Article to the contrary or any type of knife already proscribed under California Penal Code Sections 653(k) and 12020(a), no person shall knowingly sell, offer for sale, expose for sale or transfer to any minor, any type of knife as defined in Subsection (a), other than table knives. This prohibition shall not apply to the sale or transfer of knives defined in Subsection (a) by a parent or guardian to their own child.
Since SF went to the trouble of doing this, I suspect that state law doesn't cover this issue. SF also bans selling spray paint and wide-tipped markers to the little buggers.
Also, I'm sure that some stores, even if there is no local law against it, have their own policies against selling specific things to kids.
If your city doesn't have their municipal codes online, you might call the local police.
miguel