As about half the population of the US lives in cornfields and deserts where it makes little/no financial sense for them to build the cellular infrastructure to cover places that average less than one person per square mile (much of the Midwest) or where terrain would require more towers than a flatter area the same size (much of New England) it would seem that at least ¼ of Americans couldn’t get a smartphone if they wanted to. However, in urban areas, pretty much everyone over the age of 12 who isn’t a technophobe has one.
The people I know that prefer dumbphones or landlines tend to be old enough to have grown up before 8-tracks and unleaded gas, and often have worse computer skills than I had back in second grade when computers first started leaving the laboratories and entering the home. Off the top of my head, I’d say that that covers about ⅓ of what is left.
So when you take out those who can’t get 3G/4G cell coverage, the kids whose parents won’t give them one, and the old-fashioned people who are content with stuff that was deprecated a decade ago, I would guess that only about half of all Americans have a smartphone, though it may seem higher to those who live in cities where it makes financial sense to build a few towers to serve millions of people.