Repetitive behaviors, from what I gather, are things like lining up or organizing one's toys (though not all kids who do this are autistic), "stim" behaviors (fidgeting that differs from that of NTs in degree and/or kind, like rocking, hand-flapping, or doing a lot of more typical fidgets like leg-shaking, hair-twirling, or pencil-tapping in situations where NTs wouldn't), echolalia and echopraxia (precise repetitions of other people's speech or movements), and things like needing to eat the same foods, do certain things at the same time every day, etc. and being freaked out if one's routine is disrupted.
As for the "age of onset" question, I think for social communication disorder (and probably true for ASDs as well) they say that there have to be signs at a young age, but they may not be fully manifest - i.e., a clinical problem - until the kid is challenged beyond his or her limits. Which for some kids may not happen until late childhood. Furthermore, speaking as an NT and seeing this trait in myself, NT perception is often heavily dependent on situational social role cues and the significance assigned to things. We basically forget about or discount things that we consider irrelevant, or may not even register them in our memories. So if your family doesn't really think you're all that different, they will not remember anything that was different about you as a kid, or they'll blow it off as irrelevant and not mention it. Whereas by the time I was diagnosed with ADD as a teenager, my family had already long seen me as different, and so they brought up all kinds of quirky things about me dating back to my being colicky and sleeping poorly as a baby, which may have had nothing to do with the neurotic behaviors and attitudes that led to my childhood issues and the diagnosis (which I now believe to have probably been inaccurate because I no longer see ADD symptoms in myself).
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Right planet, wrong country: possibly PLI as a child, Dxed ADD as a teen, naturalized citizen of neurotypicality as an adult