Huh, that's a new one on me. At least when I was your age, I too had accounts but it was not possible to have credit or debit card (ATM card only), nor was I capable of making any sort of online transaction. Regardless, there are a few states where selling certain knives to minors is illegal, and my guess is some sellers don't have the legal expertise to find out, so they err on the side of caution. Plus the other reasons I mentioned.
General thing, like it applies all states? Absolutely not. Off the top of my head most states do not actually have any sort of written law that places special restrictions on carry for minors. There probably are a few and there are definitely local city laws like this, but they're not very common. More common are laws about bringing knives to school.
As to a police officer confiscating your knife, that's another murkier matter. Even when the knife is 100% legal to carry, some cops are....well, ignorant shall we say, and might try to convince you to "willfully forfeit" your knife without giving you a citation. I hate this behavior and it is frowned upon in most police departments, but it happens. It's obviously not advisable to argue with a cop, but always demand documentation like a receipt for any knife taken, and follow up immediately with that officer's department (same day if you can) with a parent to get the knife back. Usually ranking officers are more knowledgeable and can help sort it out.