That's how it is in Utah but I think it may be 16 years old, not 18. I can't remember for sure it's been so many years. I know it's true though because I've had people ask me for ID for buying WD-40 on one occasion and pocket knives on several. You can't buy spray paint, pocket knives, lighters, or any other flammable materials for that matter unless you are old enough.
A lot of places might not enforce it heavily but it's all in the books. So they can if they want to.