Generational intelligence test score gains in the 20th century are linked to psychometric abilities. Recent reports suggest that changes in IQ trajectories may be due to ability differentiation, resulting in domain-specific and inconsistent trajectories. Schooling is considered a main factor in the Flynn effect, but our study on a large-scale student assessment in Italy found little evidence for changes in achievement-based g overall. However, cross-temporal trajectories differed by school grade, with lower grade students showing decreasing g while higher grade students showed positive changes. These findings suggest age-dependent achievement-g differentiation and align with recent evidence of potential stagnation or reversal in cognitive test score gains.