I believe it was Don Lind, who was selected in April 1966. He was backup for some Skylab missions, but didn’t actually fly until STS-51-B in April 1985, 19 years later.

We *might* also count Joe Engle. He was also a part of the April 1966 group, but had his first flight on STS-2 in November 1981, so only 14 1/2 years after selection. However, that’s only his NASA selection - he was also selected by the Air Force in the third Aerospace Research Pilot School class in 1961, which would put his gap at around 20 years, if we’re including USAF programs in addition to civilian ones. However again, he flew as part of the X-15 program, and some of his flights crossed the USAF’s definition of the boundary of space (50 miles), but fell short of the FAI’s mark (100 km). If we count those flights, then his first spaceflight was X-15 Flight 138, which was in June 1965 - just a few years after his USAF selection.