First, there are limits under the law to what parents are allowed to do. There are also social limitations and pressure (not always for the better, but overall it is an important restraining factor).
Second, to see the power difference as age discrimination requires that the only difference be age. It isn’t. A 9 month old obviously can’t drive a car; a 5 year old obviously can’t balance a checkbook. A whole of cognitive and physical development happens between 0 and 18, all of which is necessary to developing decision making judgement.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).