The concept of mental age seems to assume that there's a certain developmental progression for maturity that follows the same pattern for everyone. You're supposed to be like this by age 10, then like this by age 20, then like this by age 40, etc. If you do those things earlier, your mental age is higher, and if you do those things later, your mental age is lower. That may be true for many NTs, but people on the spectrum don't seem to follow typical developmental patterns. We can be 'advanced' in one area while being 'delayed' in another area, even when those areas are supposed to be closely related. Socially, you could say that I'm delayed because I have trouble doing a lot of things that come naturally to NTs, but at the same time I seem to have a better intellectual understanding of the things they take for granted, which can actually let me understand better than they do if I have time to analyze the situation.
We also don't always have conventional interests, and NTs have firmly decided that certain interests belong only to certain age groups, so someone can be classified as mature or immature based only on their interests. What business is it of theirs if I like to make and play with little clay creatures and give them species and families and history? It was fun when I was 9 and it's still fun now that I'm 29. How is watching romantic comedies or football games or gossiping about celebrities a more mature activity? And what about the fact that I also like NPR podcasts and sociology and science? But then what about the fact that I still love my made up fantasy world from when I was 7? But what about enjoying documentaries? But what about liking fanfiction? But what about liking the kind of fanfiction that explores the effects of small changes over time or the implication of things that were brushed over in canon, or the way everything changes from different perspectives? And what about the fact that I liked these same things when I was 11? Am I mature or immature? Or do I just like what I like, regardless of what other people say is age-appropriate? Meanwhile, there are plenty of neurotypical adults out there who can't wait to have kids because their kids will give them an excuse to do the fun, childish things that they never really wanted to stop doing but are too embarrassed to do by themselves?
As far as emotional maturity, I'd like to see how well the typical neurotypical would handle their emotions with light shining in their eyes and sounds screeching in their ears while people get mad at them for no apparent reason and act like the things they find impossible are easy and the things they are good at don't matter. What looks like emotional immaturity may come from dealing with a world that wasn't designed for us. It also may come from feeling emotions differently.
Generally, I'm more mature than my classmates. That makes sense, because I'm older than them and have more life experience than them. However, they've never been panicked by not knowing which chair is acceptable to sit in and they have no problem picking their groups for group projects. They've also been on dates and know how to go to bars, and when presented with a new group of people they can automatically arrange themselves into new friend groups before I even realize people are doing that. So who is more mature? It's not really a valid question. I can compare myself now to my past self and say I'm more mature now than I was then, because I can see specific ways that I've improved. I can also see things that I'd like to improve in the future, so I hope that future me is more mature than current me. But comparing the mental age of people on the spectrum to NTs doesn't make sense to me. We just develop differently. It's like saying that a cat is delayed because it hasn't learned to fetch a ball or a dog is delayed because it hasn't learned to climb a tree. Help them learn to do those things if those are useful things to do, but there's no reason to compare the ages at which they learn to do them. They're just different, and we're just different. The rest of the world is just slow to understand that.
I could feel my perception altering as I read. That was awesome.