Our personalities tend to become more “mature” as we age from teens into adults, and then into older adults. But our personalities become less mature as we age from kids into teens:

Hypotheses about mean-level age differences in the Big Five personality domains, as well as 10 more specific facet traits within those domains, were tested in a very large cross-sectional sample (N = 1,267,218) of children, adolescents, and adults (ages 10–65) assessed over the World Wide Web. The results supported several conclusions. First, late childhood and adolescence were key periods. Across these years, age trends for some traits (a) were especially pronounced, (b) were in a direction different from the corresponding adult trends, or (c) first indicated the presence of gender differences. Second, there were some negative trends in psychosocial maturity from late childhood into adolescence, whereas adult trends were overwhelmingly in the direction of greater maturity and adjustment. Third, the related but distinguishable facet traits within each broad Big Five domain often showed distinct age trends, highlighting the importance of facet-level research for understanding life span age differences in personality. (more)

Many like to think that we become more “mature” as we age because our experience with life teaches us the wisdom of mature behavior. They then presume that teens would be better off acting more maturely, and should be forced to do so if necessary.  However, this “maturity as learning” theory conflicts with the fact that we become less mature as we become teens.  An alternate theory, that better accounts for the above patterns, is that we are programmed to have different personalities and attitudes at different ages, because for our distant ancestors those attitudes were useful at those ages.  Beware too easily assuming that others would be better off if they were more like you.