There’s a knack to pulling off old-timey classics. Tips for younger guys and boomers about the items worth embracing—and what’s best left in the past.

They’re skipping their lazy cargos and baggy denims for good and starting to dress sharper in well-tailored clothing like knitted polos, suits, quarter-zip sweaters. The cool boy era is over.

SEATTLE — Dylan Thompson was 17 and finding it hard to find cool, fashionable clothing for teens. So with the help of his family he started d.RT, targeting the 13 to 30 age group. The line of clothing ...