At what ages do autism symptoms first appear and when do parents first notice them, and does the timing of onset cluster right after the age of first vaccination?

The short version: autism symptoms typically show up in the first two years of life, and the timing of regression overlaps with the age of first vaccination, but none of the retrieved studies test whether the two are connected.

The retrieved studies do not directly answer your question about clustering after vaccination. What they show is the natural timing of autism onset. In one prospective study of high-risk siblings, parents' concerns about sensory and motor differences appeared as early as 6 months, with overall concerns distinguishing children later diagnosed with autism by 12 months [5]. Regression, where a child loses skills, is common: about 24 in 100 children with autism experienced early developmental regression before age 36 months, with a mean age of skill loss at about 21 months [3]. In a narrower autism group, definite language regression occurred in about 30 in 100 children [1]. A separate study of childhood disintegrative disorder found regression typically began at ages 2 to 4 [2].

The overlap is real: the first MMR dose is typically given around 12 to 15 months, and the first birthday is also when autism symptoms often become noticeable. One 1998 commentary explicitly noted this coincidence, arguing that because the age when autism first appears is also the age for MMR vaccination, the temporal association does not confer causation [4]. That is the core problem: correlation in time is not proof of cause.

None of the retrieved studies compared vaccinated versus unvaccinated children, so the clustering question remains untested in this evidence. The absence of a study is not proof of safety or harm. It means the question is simply unanswered by what was retrieved.

My call: autism symptoms typically appear in the first two years, and regression often occurs around the age of first vaccination, but no retrieved evidence tests whether the timing is connected. Confidence: not clear.