[Fiji Hill]

She's 21 (And My Dad's 52)--Webb Pierce (1967)

Firt time I've heard that one. It was apparently released on an LP, but not as a single.

[Songcraft]

Actually, I never heard it before today either.   (I was looking for a "twenty" song to continue your "17, 18, 19" list, but I couldn't find one, so I went to "21", and found that song.   It has some unusual lyrics.   Maybe that's why they never played it.)