18, technically. But if you're not 19, most people that work at smoke shops will tell you that you can't be in there. It's pretty shitty if you ask me. Although I do know of a couple places around where I live that never card.
do you mean in UT or shops in the U.S. in general? because i've never heard of the 19 thing, only in canada. in the U.S. it should always be 18