Somewhere in a methylation dataset sits a child of ten whose body is already running fast. Not sick, not visibly different from the kid at the next desk, but ticking along at a pace that a particular kind of molecular clock can read off the chemistry of their DNA. The child grew up with less: less money, fewer resources, more of the low-grade stress that comes with scarcity. And that, it turns out, is enough to show up in the body’s bookkeeping decades before any doctor would notice a...