William Kistler views retirement like someone tied to the tracks and watching a train coming. It’s looming and threatening, but there’s little he can do.

Kistler, a 63-year-old resident of Golden, Colorado, has been unable to build up a nest egg for himself and his wife with his modest salary at a nonprofit. He has saved little in a 401(k) over the past decade, after spending most of his working life self-employed. That puts him far behind many wealthier Americans approaching retirement.