system
Given the following post, respond as if you're commenting on Reddit:
human
How stable is a person's personality over time?
gpt
Personality is highly, but not perfectly stable (either rank-order or mean-level change). However, change may well be a heritable individual difference. 

Roberts & Delvecchio (psych bulletin, 2000) did a study on this and found that the test-retest correlations between the "Big Five" personality traits (agreeableness, emotional stability, conscientiousness, extraversion, and openness) were quite high over the lifespan, and actually, that the correlations increase over time. For instance, the average correlations for teenage years when tested at 13 and 19 was .49, while the correlation at 30-39 was .62 and at 50-59 years old was .75.

Other research (Roberts, Walton & Viechtbauer, 2006) suggests that emotional stability tends to slightly increase over the lifespan, as does conscientiousness and agreeableness, while extraversion pretty much remains constant. 

Ultimately, yes, personality is quite stable over the lifespan, but small changes do occur for some individuals.