We examined the effects of early life stress (ELS) on intellectual ability in 2,725 20-year-old male participants, of whom 321 were separated temporarily (mean 1.7 years) from both their parents during World War II, at an average age of 4.3 years. Intellectual ability was tested when entering compulsory military service. The separated men had -0.28 (95% CI -0.39, -0.16), -0.13 (95% CI -0.25, -0.01), -0.18 (95% CI -0.29, -0.06), and -0.19 (95% CI -0.30, -0.07) SD units lower verbal, visuospatial, arithmetic, and composite score of intellectual ability, respectively, compared to non-separated. Participants who were separated for more than a year and between two and four years had the lowest ability scores. ELS showed most consistent associations with verbal ability, but were not limited to it. Children beyond infancy and before their school-age may be the most vulnerable to the adverse effects of the ELS. (Contains 2 tables and 3 figures.)