The onset of traits is early childhood, but as others have mentioned, and as stated in the DSM, the traits may not be apparent until later on. If you, your parents and other close family members and friends can look back on your childhood and not see any traits, then an ASD is unlikely.
My childhood traits were in no way obvious, but looking back now, one could connect the dots. I began reading books back to my grandmother in full at age 2, which is around the time my special interest and sleep difficulties began. In elementary school, I was very bright and was placed in a gifted program, but my teachers frequently reported that I spoke out of turn in class. I didn't like cartoons, often choosing to watch The Price is Right or old musicals. I was probably just seen as 'goofy' and 'bright.'
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Aspie Quiz: AS - 141/200, NT - 77/200 (Very likely an Aspie)
AQ: 34/50 (Aspie range)
EQ: 32 / SQ: 68 (Extreme Systemizing / AS or HFA)
Diagnosed with AS and Anxiety Disorder - NOS on 03/21/2012