The Estonian NEO-FFI was administered to 2,650 Estonian adolescents (1,420 girls and 1,230 boys) aged from 12 to 18 years and attending 6th, 8th, 10th, or 12th grade at secondary schools all over Estonia. Although the mean levels of personality traits of Estonian adolescents were quite similar to the respective scores of Estonian adults, there was a developmental gap in Agreeableness and Conscientiousness. Three of the five personality dispositions demonstrated a modest cross-sectional change in the mean level of the trait scores: the level of Openness increased and the levels of Agreeableness and Conscientiousness decreased between 12 and 18 years of age. Although the five-factor structure of personality was already recognizable in the sample of 12-year-old children, it demonstrated only an approximate congruence with the adult structure, suggesting that not all children of that age have developed abilities required for observing one's own personality dispositions and for giving reliable self-reports on the basis of these observations. The self-reported personality trait structure matures and becomes sufficiently differentiated around age 14-15 and grows to be practically indistinguishable from adult personality by the age of 16. Personality of adolescents becomes more differentiated with age: along with the growth of mental capacities the correlations among the personality traits and intelligence become smaller.