I would say to anyone who believes that autistic kids grow out of autism, that kids grow out of their clothes and shoes (I was wearing small men's shoes when I was eight), not their autism. It's the same as people saying that kids "grow out of asthma." Okay, that's a bit more nuanced. Some four year old kids, for example, have asthma, and, as long as it's not exercise-induced asthma, some of them, as they swim, run and exercise more can increase their lung capacities strength. Some of these kids may outgrow their asthma, but it may recur when they enter their 40s, or so. Also, some kids, if sent for allergy tests, and the triggers are identified and they are removed, can find their symptoms improve. AND, some kids, who are say, later found to be gluten intolerant AS WELL, may find that their asthma symptoms improve once they go on a gluten free diet. I'm not saying that all parents who have asthmatic kids should put them on a gluten free diet, but I remember reading a story in a coeliac magazine, about a man who had really bad asthma and his doctor told him to exercise. The problem was, he'd go for a run, and partway through the run, he'd get the urge to poop. Once he was tested for coeliac disease, he removed gluten from his diet, as he tested positive.

What can happen is, some autistic kids can learn to speak up for what they need, or avoid triggers, and that can make them appear as though they've outgrown it, when they haven't.