It's true that autistic people have the brain/mind of a child even when they are older?

If you've only ever been exposed to the depiction of non-verbal, extremely sensitive-to-stimuli, routine-oriented, potentially-violent, autistic kids, that you see on TV, you could be forgiven for thinking this.

But, no, that's only an exceptionally small window of what autism is. Most cases of autism aren't so severe, and most people learn coping strategies as they grow up that let them live relatively-normal lives. Even severe cases can do this, but it tends to take more time, and more focused, expert care.

You probably know autistic adults, and aren't even aware. Or, potentially, you just write them off as being loners or not good with people or having some other character flaw, that ultimately stems from their coping mechanisms.

As an autistic person, no. Not in any literal sense anyway.

It’s a spectrum for one thing, you’re not going to have any broad generalizations like this across the population

Autistic adult here:

I do feel like I have a more childlike appearance sometimes, less so in recent years. I think that because of the seriously messy place my mind was in from the very start, it took me longer to interact with other people enough to develop the social awareness I needed to "fit in", and not "fitting in" is often equated with being childlike, IMO. I still occasionally mutter to myself in public, have odd movements and posture, and generally act in a way that diverges from the social norms of the people around me, for better or for worse. Medication has changed all this around in ways that are too complicated to get into in one comment.

My mind never stopped developing. My brain chemistry changed as I went through puberty, and then through adulthood when the prefrontal cortex starts doing its thing. I kept gaining new knowledge from my surroundings and my peers and that changed how I thought about things on a basic level. There are certain specific areas in which I was always considered "more mature".

My experience doesn't necessarily reflect those of other autistic people who've had different hands dealt to them. I'd be happy to answer any other questions you have.

No, but I can understand why you'd ask that.

Statistically, an autistic adult is more likely to have "childish toys" like dolls and stuffed animals than a neurotypical adult. This, and other traits that could be described as childlike, could lead someone to believe that they have a child's mind, but my own personal theory (not professional at all, just some personal experience with autistic folk, and a suspicion that I might have a touch of the 'tism myself) is that they just don't care about the norms as much. Most of us decided to give up childish things even though we still enjoyed them, we just felt pressured to "grow up", but autism makes it harder to understand social norms, so they just keep doing what they do because that's what they enjoy and how you feel about it doesn't really matter.

No. I'm in a community of about 300 people on a discord server (we play games and such together) and quite a few of them are autistic, and for most I didn't even know until they told me they were. The ones I did notice it was more a pattern of fixation behavior and such than 'wow that 30 year old has the mind of a 12 year old' or whatever.

It's true that people can be children at heart, and there's nothing wrong with it. They have jobs, but at home they snuggle plushies and bathe in slime because they bought those things with adult money.