According to the age differentiation hypothesis (ADH), g becomes increasingly invested in specific abilities with age: with increasing age, g should account for a decreasing proportion of the variance in any particular cognitive test. However, there is no conclusive evidence concerning this important topic. Data from the Italian, Spanish, and American standardisation samples of the WPPSI and the WISC-R were analysed in the present study for testing the ADH. No changes in the proportion of variance associated with g in the 4–16 age range were found. This pattern does not change depending on the country. Therefore, the ADH is rejected: g seems to be very stable across age, and, thus, it seems to be an equally important component of intelligence test performance from early childhood to late adolescence.