I tutored a boy who was severely dyslexic. He was called for an aliyah, and gave a drash, as was tradition in the congregation, and he memorized his maftir, and had someone else point to it. The portion was not the one closest to his birthday, but was chosen because the maftir was very short. He did not read the haftarah.
That’s an example of the adaptations made for someone with, in the spectrum of disabilities, minor ones. His dyslexia was severe as far as dyslexia went, but nothing was wrong with his mental faculties.
I know another young man who is the son of a rabbi, and marked his bar mitzvah at age 25. He is autistic. His father said he wasn’t sure the day would come, but he was confident that his son fully understood at this point what it meant to be responsible for the mitzvot, and he was truly a bar mitzvah, even though he was not one at 13.
This underlines cmkeller’s point that you don’t “have” a bar mitzvah, you “become” one. This same young man also eventually got a driver’s license, but not until age 27. He’d had a learner’s permit since age 23 or 24, but wasn’t ready to be driving on his own until he was 27, and a lot of that was his own input.
Anyway, someone not capable of performing ANY duties of a bar mitzvah will not become one. If someone does not understand why we fast on Yom Kippur, or even that the fast is a tradition everyone participates in, it is cruel to withhold food from that person for 25 hours. I don’t know anyone who would do it. I wouldn’t. An autistic meltdown because the person has no idea why he is being denied food isn’t really any different, as far as whether it is OK to feed the person under Jewish law, from a person with hypoglycemia who passes out.
Additionally, someone who will always need another person to, for example, make sure his food is kosher, is not a bar mitzvah, no matter how old he is.
That said, a first aliyah is a simcha, a thing to celebrate. It usually happens around the time you become bar or bat mitzvah, but it can happen later. I know lots of women who had their first aliyot as adults, and had big simchats. Nothing wrong with that. It’s sometimes called an “adult bat mitzvah,” but that is a misnomer. I understand why people call it that, though. But the only people who literally become b’nei mitzvah as adults are converts.