I've been through improvements and some "regressive" times (losing skills I totally had before) since I was born.
Under circumstances suiting an autistic person, they can always learn to improve. I'm not claiming that such a person doesn't exist (especially because the ability to learn/learn fast definitely is restricted a lot by profound MR which can co-occur with autistic symptoms, though even someone like that learns of course), but though I often hear from therapists/specialists that it is super rare for any "high-functioning" or "low-functioning" autistic child or adult to noticeably improve (and that I'm a total exception, supposedly) I have yet to meet an autistic child or adult who does not improve in many areas that are affected by their autism if 1) together with therapists or parents they got a form of meaningful communication working (speech, cards, signing) and who 2) are actively taught stuff. After all, there's no way to improve if nobody offers you the opportunity to learn or when you're taught in a wrong way by which you cannot learn because of your disability. (Imagine a person with a hearing-impairment hardly hearing you but you keep on talking... how would they know what you're talking about unless they came up with lip-reading or something all on their own?)
Basically, almost all of those autistic people who are said to show "zero" improvements, they tend to either not have access to a form of communication that suits them, they learn very slowly and fewer things than most people to co-morbid conditions but they do learn (and maybe people don't pay attention enough to their improvements) or they're at sub-par institutions where they get taken care of 24/7, might adjust and feel comfortable but aren't taught to be more independent or even get frequently discouraged from gaining independence because there isn't enough staff available to help them with it and keep an eye on them.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett