As far as I know, the CPSIA doesn't include any age restrictions on who you can sell what to. The law is based on what's considered safe for the age of the intended user, not the purchaser.
That's one reason why warning labels are required on packaging & not just displays. The info needs to stay w/the product after the customer leaves the store with it. It's also why people can't sell hazardous baby products on the theory that the parent is buying it, not the child, so it's up to the adult to decide how to use it.
Unless you're selling alcohol, tobacco, guns, porn, etc. (varies by state- I was surprised when I moved to CA that I needed an I.D. to buy spray paint!) you don't need to ask for proof of age.