Like with everything around autism, it differs. For me the first signs started to appear between 3 and 5 (or so they tell me) but bigger problems arose around 13.

(Fourth throwaway I have to create for 6 posts despite 10 upvotes. The captcha is a real PITA to get through and by now I have to tether 3g to comment anything at all...)

Within the first year. There is a list at the bottom of the article, and there have been studies that have shown some signs appear as early as 3-months.

When my twins were born, I knew pretty quickly that my daughter was different. While her brother was all eye contact and cooing, she was always in her own world. Without a neurotypical sibling, it might have taken much longer to spot.