Matt Thurmond seems like a poster child for tech-forward millennials. He runs an AI-assisted platform for mortgage professionals. He leads a nonprofit that connects longevity researchers, investors, and startups. He was the copresident of a technology conference at Harvard, where he got his MBA.So it's a little surprising that Thurmond is almost never on his phone.Count him among the "appstinent" — one of a growing number of Americans, mostly millennials and Zoomers, vowing to live a life free o...