Blarg:
>>>>>>>1. Having the spankee be 18 and the spanker be older - perhaps just a little, or perhaps much older. <<<<<< Have you ever considered the practical difficulty in spanking an 18-year-old who is shocked, awed, appalled, and as a result unwilling to be be spanked?
2. >>>>>Describe the adolescent as being physically smaller than the adult. You could have an 18 year old boy be say 5 feet tall, quite short for his age, and is perhaps living in the home of a 40 year old landlady who's 6'2" and stout. Describe him as young-looking and often mistaken for younger.<<<<<
So here you have physical force overwhelm an unwilling victim. Sounds pretty abusive. What happens after? Like not just after the spanking, but after the shocked, awed, appalled victim dials 911?
3.>>>>>> Put the spankee in a subordinate position in some way to the adult spanker. Maybe the adult is their foster parent and they're still living in their home. Perhaps the adult has taken them into their home as a host family situation. Perhaps the spankee is an 18 year old runaway they took in. Perhaps the adult rented a room in their home to the adolescent, who then loses their job or otherwise becomes unable to pay rent; the adult lets them stay anyway, but gradually it becomes apparent they don't consider the adolescent an adult free to do as they please when the adult is putting a roof over their head, food on their plate, and perhaps even clothes on their body.<<<<<< So if beingspanked is "part of the bargain" of being cared for, getting spanked wouldn't be that shocking.
4. >>>>>Have younger adolescents in the story; make it clear in various ways that the adult considers the 18 year old a member of the category "kids" rather than "adults"; have the adolescent spend their time with the younger adolescents and treated as one of them by the adult.<<<<< See previous answers.
5. >>>>>Have the adult treat the adolescent as a kid in various ways, not all of them embarrassing or unpleasant at first glance, or at least unrelated to discipline. Maybe the landlady with the 18 year old tenant who can't actually pay rent has him move into the same bedroom as her 12, 13, and 14 year old sons; he spends most of his time hanging around them anyway. She notices his clothes look ratty (he has no family, and bounced between foster homes) and takes him with her sons to buy them all clothes. He's small, so she buys him clothes in the boys' department with her sons.<<<<<<Sorry, but the spanking landlady fantasy is utterly unbelievable.
6. >>>>Establish that he isn't too old to get punished. Maybe he oversleeps one morning and almost makes the family late for an appointment one day, so when his landlady says he needs to go to bed the same time as her sons, he's embarrassed at having a bedtime like a kid, but is too embarrassed about making them late to argue about it. Maybe he messes up a few times and his landlady grounds him; he's embarrassed and thinks he's too old to get grounded, but again feels vaguely like arguing would only make it worse - or maybe he does argue and the one day grounding turns into two. The thing to establish is a feeling that there's nothing he can do about it. It's out of his hands, and he's firmly in the role of one of the kids in her eyes.<<<<<<<<
It's not about age, it's about resistance. All the intended victim has to be is big enough to resist, and savvy enough to report the incident as abuse. The only way around this sticky wicket is to have the victim NOT be shocked by the prospect of a spanking but......if not want it.....at least see it as something not so unusual or inappropriate. But then they'd hardly be shocked if one comes their way.