I identify as a little. When I am little me, I'm being in the headspace of a 9-12 year old. I often felt uncomfortable with the term Adult Baby, as I don't enjoy babyish stuff at all, except for diapers. I don't enjoy plushies, don't want a pacifier or sippy cup or bottle, and don't want to crawl around or be helpless.
I do enjoy playing with toys like cars, soldiers or Legos, and like colouring in. It makes it a bit hard to incorporate diapers into this age set, but it can be done. When I do play as a little, there is always some element of wetting involved. This is where the diapers can come in. I may be distracted as I play, wet my pants, and be diapered as a punishment. Or I may be a bed wetter who wears a nappy to bed, but has either not gone to bed yet or has woken up dry but is still wearing.
I'm lucky to have a girlfriend who likes age play as well, but for her it's a younger more babyish age. She likes to be 3-4, and be diapered and cared for, and likes to suck her thumb. But we can be flexible to suit each other's playtime. When we play together, we take turns indulging in each other's fantasy. We can play as big kids who might be accident prone, then I can change her and she can be as little as she wants.