For a moment, I fell under the spell of Bryan Johnson. Bathed in early-morning sunlight, the 46-year-old L.A.-based tech centimillionaire and longevity celebrity didn't look much younger than his age, although he claims to have the wrinkles of a 10-year-old and organs that are several years younger than his lifespan. We were standing at the Temescal Canyon trailhead in Pacific Palisades on Jan. 13, ahead of a Johnson-sponsored "Don't Die" hike, one of many organized across the world that day and the only one hosted by him. Of the 500-plus people who had RSVP'd for the L.A. event, about 200 showed up. Some had slept in their cars to make it.