I would think that there's little difference between feeling "little" or being AB. How far down the age scale we regress is also part of a spectrum, but we end in the same place, feeling either like an infant, toddler, or pre-schooler. The more important aspect is to find times that are appropriate when one can play a little bit, wearing diapers, baby clothes, using bottles or sippy cups, etc. As is said in the introduction thread, "diapers don't rule our lives". I would suggest that if regression rules your life, then yes, you have a psychological problem that should be addressed.
I'm a strong believer that as humans, we are meant to be independent, that we can support ourselves and eventually, survive on our own. When we are first born, everything must be done for us by our parents, otherwise, we would perish. But as we grow, we learn to become independent, learning life skills from many sources, parents, schools and life experiences. To not grow, mature and eventually becoming self sustaining, one would have to conjecture that there is a mental disability involved.
Some people suffer from depression which is so severe, they simply can't function. I knew a few like that when I was in college. I watched them fail out and I felt sorry for them. I wished I could have helped them, but I didn't possess those skills, nor did the school services provided to them, though they were minimal. I was one of those who suffered from depression but I found ways to escape from it and at least, get by. One thing that helped me was that I had friends and I was able to socialize.
That brings up the other half of the original thread. Not being able to socialize with others is another indicator of having some sort of mental disability. As humans, we tend to be social animals. We have survived first by being tribal. Those evolved into communities. We pooled our talents and abilities so that our collective needs were met. To not be part of the community is damaging. I believe we need each other, not just for an exchange of goods and services, but to maintain a level of happiness. When we turn inward and avoid others, we can fail to thrive, and that becomes serious, and we then need some sort of outside help.