Big Five personality model

Maturity principle

The maturity principle is the observation from longitudinal studies that, between ages 20 and 40, people on average become more Conscientious, more Agreeable and less Neurotic.

The changes are gradual and the rank-order between individuals stays high: if you were more Conscientious than your peer at 25, you probably still are at 55, even though both absolute scores drifted upward.