The age gap

The perceived wisdom of heterosexual marriage - at least in the UK when I was a lad - was that the husband should be slightly, but not excessively, older than the wife.   Exceptions abounded all over the place, but were always seen as being exceptions to the rule.   A wife "too young" was either indicative of a gold digger in action, or a dirty old man getting his wicked way, depending upon your point of view.   A wife "too old" was usually indicative of a mommy's boy achieving his ideal partner.

And this seems to have spilled over into spanking fiction:  the participants in an adult relationship that is SSC are expected to be of about the same age:  and in the genre of erotic spanking, in their youthful prime.  When the shorthand M/F is used, he may be excessively dominant and she excessively submissive, but woe betide you if you make it clear there is a huge age gap between them.   Particularly if the action after the spanking turns into a sex romp.

The same sort of unwritten rule seems to apply to discipline, in real life as in spanking fiction.   A male  disciplinarian is expected to out-age a female penitent, by some degree, however small.   M/F is back to our heterosexual marriage rule, M/f is dad whacking his teenage daughter and M/g is a teacher whacking a young schoolgirl.   (Point worthy of note - any /g tale is fraught with potential difficulty and having a very young girl get spanked is more likely to get you hate mail than a growing band of followers) - of course, your mileage may vary.

When we have protagonists who are F/f and F/g the age gap is instantly obvious.   F/F has some interesting age considerations in play,  You might find that if the two ladies are of similar age, you have no need for narrative to explain their relationship and that the bigger the difference in age there is, the more detail you have to deal out.   However, should your tale have some twenty year old young woman taking her hairbrush to a forty year old's bottom, you really will have to spend a lot of time showing how such an unlikely outcome came about.   On the other hand, it does not need anywhere near as much work when a 40 year old teaches a lesson to someone in her acquaintance who is only 20 years old.

The scenarios of M/m, M/b, F/m and F/b clearly have no problems dealing with the inherent ageism we spankos seem to have.    And when it comes to M/M and F/M, we are in area where I have no personal experience or observation to share with you.   I suspect that the dominant party "ought" to be about the same age or slightly older than the submissive one, but really don't know for sure.   But even as an outsider looking in, I think that if you write about a 20 year old young woman taking a cane to a 40 year old male, you will need a lot of set up to get the reader to be comfy with the situation.