My little age is something that varies wildly. When I really want to get into little space, I go down to about 1 year. A single year is young enough to require diapers, crawling, paci, and baby TV shows. But my little space generally encapsulates the using of the diaper and specifically how it is used.
I feel like I need to modify my little age based on what I have available and how I want to act. Toddlers have some continence, which gives me the key to run around, playing, then suddenly stop to put a load in the seat of my diaper, then keep playing as if I didn't even stop or mess myself. This, while still having tiny squirts of pee as I usually do when wearing a diaper. I like toddler mindset better than baby mindset for messing because it's harder to act totally bowel incontinent and letting what has developed simply slide out without clenching. As a toddler I can power through the whole mess.
Sometimes I only have pullups available, so I may act like an older child who has potty problems and is young at heart. Toys, shows, catching myself last second for the potty. That kind of thing.
Example is now, I don't have much to throw around on diapers, so I have some Tena Underwear. I wear one on the premise of "just in case", acting as if there had been a time where I didn't make it and I don't trust myself without a diaper anymore, at all. Extreme, but functional for when I want to destress. This doesn't involve using the diaper, but motivates me to go to the potty more often.
So, conclusion, my little space revolves around the diaper first and foremost and how I use them. Further regression is a bonus applied on top of that for further destressing.