Here we test the developmental theory of sex differences in intelligence using latent modeling. The theory has been recently updated (Lynn, 2017). Based on men's greater average brain volumes and premature maturation in women, the theory predicts an intelligence advantage for women (or null sex differences) in early adolescence, but an advantage for men in late adolescence. The findings reported by Lynn are consistent with the theory, but basic measurement issues were not met. Are we taping the same construct regardless of sex and age and can we provide sound conclusions? We considered 10,335 individuals (4992 boys and 5343 girls) within an age range of 12–18 years. They completed the subtests included in the TEA Ability Battery (BAT-7). Theta scores for each subtest were estimated using a three-parameter logistic Item Response Theory model (3PL IRT). Results fitted properly to invariance criteria across age and sex. Latent estimates for the general factor of intelligence (g) were used for testing the theory. The observed findings were in agreement with the developmental theory: there were null sex differences at 12 years of age, but there was an average difference favoring boys equivalent to 5 IQ points at 18 years of age.