Background: Delays in development are a fundamental feature in diagnosing autism spectrum disorders (ASD). Age of language acquisition, usually obtained through retrospective caregiver report, is currently used to distinguish between categories within ASD. Research has shown that caregivers often report children as having acquired developmental milestones earlier or later than they were actually achieved. The current study examines the extent to which this phenomenon, referred to as "telescoping," impacts retrospective reports provided by caregivers of children with ASD. Methods: Participants were 127 caregivers of children referred for possible ASD or non-spectrum developmental delay. Caregivers were interviewed when children were 2, 3, 5, and 9 years of age. Caregiver-reported ages of first concern, language and non-diagnostic developmental milestones and interviewer-estimated age of onset were compared over time using linear models. Results: Significant telescoping of language milestones resulted in more children meeting language delay criteria as they grew older, in spite of original reports that their language was not delayed. There was little evidence of consistent telescoping of caregiver-reported ages of first concern, daytime bladder control, and independent walking. With time, the interviewers' judged ages of symptom onset increased, but remained prior to age three. Conclusions: Telescoping of caregiver-reported ages of language acquisition has implications for both clinical diagnosis and genetic studies using these milestones to increase homogeneity of samples. Results support proposals to remove specific age-based criteria in the diagnosis of ASD. Telescoping should be considered when working with any clinical population in which retrospectively recalled events are used in diagnosis.