It differs for me. When I was a young child, until about age 13-14, I always felt older and more mature than my peers. I was more at ease interacting with adults, and found conversing with adults to be much easier. I followed rules and understood many adult concepts like money, responsibility, accountability, freedom, laws, governments, etc. This stayed true until I was about 14 years old. At that point, I became highly confused. I was still watching cartoons and kid shows and was still entertained by that. It wasn't that I didn't like adult shows, in fact some of my favorites were old reruns of Get Smart, Dragnet, Mary Tyler Moore, Cheers, etc. I had always watched and enjoyed many adult programs in addition to my cartoons. I just noticed suddenly people thought watching cartoons was weird or inappropriate, and I didn't quite get why. I still played the same games I'd always played, with my legos or hot wheels cars, but again, this was suddenly "not OK". To the extent I socialized with anyone, I socialized with my younger cousins and family friends. I'm pretty sure my extended family started to think I was somewhat "off" at this point.
I actually didn't understand this at all until I had a better grasp of psychology (from classes in HS and college). When I understood the processes intellectually, it made sense. I still watch cartoons (mostly anime and Family Guy/South Park type shows). I still play video games, but now many adults do and it's not that unacceptable. I now feel much younger than I actually am (31), primarily because I don't feel I've changed very much mentally while others have. I don't expect much will change with age. I'll age physically, but mental "aging" as I understand it, is largely a process of changing social expectations and roles in society, which my aspie brain has a limited ability to even understand. In many ways, I'm no different than I was at 12, when I feel my intelligence was mostly developed, even if my experience was still limited.
My theory on this matter is that without feeling peer pressure and feeling the instinct of most social animals to be part of the "pack" or the "herd", we simply don't change much mentally after our brains are fully developed. Without this instinct, there's no primary motivation, and even if we do feel some impulse to be like others are, we have a limited ability to understand intuitively what's expected of us. If I try to behave "age appropriate", it really is simply an approximation of what I intellectually understand to be the expected psychology and behavior of a typical person my age. I can do a very good approximation. I can fool most people into thinking I'm normal for a limited period of time, but it's like flexing a muscle, I can't keep it up for long, and if I try to the energy drain is unsustainable. I'm fortunate in that I communicate well enough in matters of rationale, logic, and reason to function in the adult world. I do wonder, however, if I actually possess the ability to form emotional relationships outside the familial ones I inherited from early childhood. It gives me pause to wonder how we define such formless, insubstantial things as relationships and how something so easily understood by NTs intuitively so thoroughly evades the absolute logic and rationality of my aspie mind.
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Self-Diagnosed Dec. 2010
135 Aspie, 65 NT--Aspie Quiz
AQ 40
BAPD--124 aloof, 88 rigid, 83 pragmatic
EQ/SQ--21/78--Extreme systematizing