Blarg:
* Going undercover as somebody significantly younger. "Back to school" and they think the protagonist is school age.
...
And then there's my favorites, the various "finding out you aren't too old to get it" subgebres:
* Maybe the spanker knows it; and the spankee doesn't know until it happens and they find out the hard way.
For the "Back to School" situation, I'd highly recommend the serial
"Sandy's Adventure" by profken, it deals with a thirty-year-old woman who pretends to be of high school age in a socially conservative area, one wherein 'Spare the rod and spoil the child' is still a widely practiced child-rearing philosophy, even in disciplining teenagers.
Regarding the "Not Too Old for Spanking" account, this isn't really 'in my wheelhouse,' yet I did write one story,
"Summertime Reds", which deals with whether or not an eighteen-year-old boy is too mature to be spanked by his former babysitter.
I'm certain there are numerous others in this Library...

--C.K.