Store policy & local conditions...
Spray paint. I got carded for buying spray paint.
The town 20 miles away (with an ethnic gang problem) has an ordnance, and all the stores have the spray paint locked up. The Walmart also keeps all the video games and movies locked up.
The town 20 miles away, the other way, doesn't. And they (Walmart included) don't.
Walmart always rings up to ask for my age for a number of products. Rarely do clerks actually ask for my birthdate. Once in a while they do ask, if I'm over 17/18 or 21, jokingly. Most just glance at the gray hair and white beard and punch something into the cash register and go on ringing up items.
I never get insulted by being asked, because I recognise its just procedure, and the clerks can actually get in trouble (technically) if they don't ask. Usually a level look, and a raised eyebrow is plenty to send them into embarassed activity.
Many places have either laws or local ordnances prohibiting sale of a knife to a minor (although the limit is sometimes lower, 16, or 14). And in more rural America, even these rules are often ignored by local merchants. But we don't advertise that fact, after all, in an era when you cannot carry a set of fingernail clippers on an airplane, letting a 9 year old buy a pocketknife is barbaric...isn't it?
The town that cards you (as an obvious adult) for buying spraypaint is not going to sell a child a "dangerous weapon" like a pocketknife.
Alles muss in Ordnung sein! its not just for Europeans anymore!