My son attended a school for autistic children for a couple of years a while back, and they had threads that had the appearance of occupational therapy - teaching kids to cope with aspects of their autism that showed up as limitations.  Our county also had such programs (don't know if they do now).  From where I sat, it appeared that these programs could teach the kids coping strategies that would, in some circumstances, enable them to integrate well into our society.  So for me, the operative phrase in your question is "no longer seem to."
 
I don't have the professional training to say one way or another if the visible signs being on the austism spectrum can go away over time (I do know from firsthand experience that they change) but I do know that we can learn behavior patterns that make us seem normal to most people.
 
Education.com does have a lot of articles on autism, and one editorial piece (link below) on how diet can affect it.  This article and the comments that follow it, do say that autism symptoms can diminish as a function of diet.

I'm sure that this is not the case with all children who are diagnosed on the autistic spectrum; but I have a son who was diagnosed in his preschool years as a high functioning autistic. Some pediatricians label him with Asperger Syndrome. Whatever his true diagnoses is, he is somewhere on the spectrum. He is now 14 and while he may have some "off" days he is nowhere near the same state that he was in a few years ago. From the age of 2 up until about the age of 10, he only had one friend (another child with ASD) and he appeared eccentric among his peers. I say eccentric for a lack of a better word. What I mean is that he definitely stood out as being different and much of his behavior was inappropriate.
 
I made the decision to home educate him to keep him from ridicule and to focus on his talents and gifts rather than send him into a situation where the focus would be on his deficiencies. I feel that staying home has been instrumental to all of his various successes because he moves at his own pace for the most part; he has obligations and deadlines that I set for him to give him a taste of the real world-- but the goals are realistic and obtainable in order to feel the accomplishment. To date, he is one year ahead in high school and even attends a local community college to tackle classes that are required of him in his high school program.
 
He is a talented musician and he has goals that he has set on his own. He desires to have his associates degree simultaneously with his high school diploma and immediately apply with the state parks system to become a ranger. I'm not sure if he would be where he is today without one to one teaching, outside volunteer experiences, accountability to community college professors and wonderful mentors like his gym teacher, music teacher or youth pastor.
 
I know it may not be realistic for everyone to home educate; but I will say that I firmly believe that home education is close to perfect for the child with ASD. Children with ASD need to move at their own pace and in their own time and feel success along the way. Home education has provided that. He has not been lost in a classroom, he has not been discouraged-- only encouraged, and he has not suffered the ridicule that even the best and the most "normal" children endure on a daily basis in a traditional brick and mortar school setting.
 
I hope that you find encouragement here. Autism has been more of a gift than a problem in our family and when the perspective changes from that of a negative view of ASD to a positive one- many doors open up and the possibilities for the student are endless. Cheer your child on and find the blessing in the midst of the challenge.