They used to treat me like a child because I would smile a lot and let my guard down and say whatever came to my mind. I would not let them know what I wanted to do, but I would defer to whatever they wanted.
Now (I learned this in the past year - I'm 45) I don't smile much (I practice by pressing my hands against the muscles around my mouth), don't say much except things that have some meaning, I tell them what I want to do and do it. The result is they don't treat me like a child.
I didn't tell them not to treat me as a child by speaking words to them. I told them by my actions. This latter kind of "telling" is more meaningful to NTs.
It's a win-win, since now my family claims I'm more pleasant to be around. Give it a try.
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44/50 AQ
Formal Diagnosis ASD-1
5'-10" 175 lb. 225 bench, 290 squat