I was A.F., ‘63 - ‘67 and the highest I saw anybody go in 4 yrs. was E-4, with the only exceptions I knew of was the U.S.A.F. band Washington, D.C.
Band members entered the band as E-5s and very rapidly made rank. Not uncommon to see 20 year old E-9s.
I made E-4 in 4 years, while many, many got out after 4 years as E-3s.
Well, they had incentives. I came out of Basic as an E-3 because of college. I got an early education in what the military would be like, so declined 2 or 3 chances to go before a below-the-zone board for early promotion, survived 2 or 3 suspended busts and got out a Sergeant.