The person in question also looks like she’s in her early 20’s. She had a chronic condition where she was basically in a waking coma but wasn’t “experiencing” stuff for a decade so it’s as if time had jumped from her POV because she has no memories of that period.
This gap is so disorienting for her that she identifies her age as 22. Being told her actual birth year causes confusion and distress, as it clashes with her sense of time and personal experience.
By the time you’re in your mid 20’s you stop caring about how old people are. I hang out with older and younger people alike; who they are as a person is far more important than when they were born. I stopped keeping track of my own age by my late 20’s, and now I have to do quick mental math every time someone asks me how old I am.
Now, I don’t have a weird condition that makes my mental age the equivalent of someone in their 20’s, but I imagine that someone who did would have all the more reason to want to hang out with similar-minded people. The real question that should be asked is why do you think it makes so much of a difference when everyone is well into adulthood, even by self-proclaimed mental age?