In short, you don't really know, and it can be more difficult to naturally regress when you're constantly wondering about what age you might be.
At first, I had no idea. I've always enjoyed kids things, but I never really put a number on it. When I discovered that being little was a thing, I started to experiment. I wore some goodnites. Got a cheap pacifier. Got my old stuffed animals out. And went from there. I weeded out what I didn't like and stuck with the stuff that I did. I always make sure to revisit the things that I didn't care for, just in case my opinions have changed. Then, once I figured out what I liked, I put a number on it. I figured that kids usually use diapers until they're 2 or 3, so, I set that as my minimum age. I don't particularly enjoy being helpless or speechless, which is why 2 is my minimum age. Then I looked at all the other things that I like. Older kid toys like Lego, watching movies that aren't animated, eating "bigger" foods. And I put a cap on that, which, for me, is around 8 or so. Now, keep in mind that these aren't hard limits in any way. Sometimes, I like not being able to talk or walk, and so I feel 1. Some days though, I want to draw detailed pictures and play more advanced computer games, and I feel 10. It's very dependent on your mood.
Don't limit yourself, experiment, and have fun