they take an IQ test.
adverage IQ is 100 (with a 'grace' frame of 90-110. if you are between these numbers you are right on target)
mild mental diability = 50-70 (function of a 12 year old)
moderate = 35–40 (6 year old)
(some tests will say moderate is 35-55. so 50-55 is a grace period)
severe= 20-35 (grace period is 35-40)
profound= below 20 (some tests below 25)
autistics can be in most any of these ranges.
autistics can also be savants- one gifted abilty that surpasses even people with genius IQ (for instance, sometimes there math abilities can only be matched with a computer) these abilities are innate, they are not learned.
(you might want to look up the videos on youtube)
autistics can also be Ausburgers. they have basic IQ but low emotional functioning, short attention spans, inabilty to read others emotions/social ques, and sencitivity to sound/light/ect
many autistics are just in their own world and can't take a test. it may not be that they can't open a door- but they don't even realize the door is there. they are all very low functioning.