Probably Deke Slayton. He was selected for the Mecury Project in 1962 but was grounded due to an atrial fibrillation until a flight in 1975 on the Apollo-Soyuz project.

I'm on a mobile interface so I can't do this now, but perhaps contrasting these two lists will help (both via Wikipedia) :

1. List of astronauts by year of selection.

2. List of astronauts by first flight.

I’m looking at his Google results and I see 1975 for the ASTP flight, so 13 years. However, I think Jeanette Epps has him beat, because she was selected in 2009 but will make her first flight next year, leaving a gap of 15 years.

I didn’t even know those were on Wikipedia, thank you very much

Good point. But the resource on another comment says Deke was selected for Mercury in 1959. If that's correct, that puts him at 16 years.

60 years. Wally Funk was one of the Mercury 13 and was selected in 1961. The M13 were women who underwent testing and training, but this was not an official NASA program. She flew on the first New Shepard flight in 2021.

That one was correct. Although someone suggested Wally Funk, and I think she’s the true holder of the record

Thing is, the “Mercury 13” weren’t officially selected for anything.

I wouldn't count her since the Mercury 13 were never selected as astronauts.

Point taken. Slayton probably holds the record for all who went through government agencies and trained for an orbital-class mission.

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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.

That might be it