Personality is often assumed to be stable over time, based on the theory behind psychological traits (read more about the difference between traits and states here). In short, we are born with different temperaments which continues to affect our behavior throughout our lives. 

There have been many studies about the heritability of personality traits, and some of them are summarized in a meta-analysis by Vukasovic and Bratko (2015). Combining results from more than 100,000 participants, they found a strong influence on genes on personality. For the five factor model of personality, between 31 % and 41 % of individual differences in personality is due to genetic effects*:

Neuroticism (inverse of Emotional Stability): 37 %
Extraversion: 36 %
Openness: 41 %
Agreeableness: 35 %
Conscientiousness: 31 %
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These findings indicates that our personality is strongly determined by our genes. Since our genes don't change as we age, this should make our personalities stable as well. At the same time, the effect of environment on personality is even greater (60 %). What does that tell us? Does our environment affect our personalities more in a young age compared to later in life, for example?

Other studies have investigated the change in personality across the life span. A meta-analysis by Roberts, Walton and Viechtbauer (2006) reports that:

'... people increase in measures of social dominance (a facet of extraversion), conscientiousness, and emotional stability, especially in young adulthood (age 20 to 40). In contrast, people increase on measures of social vitality (a 2nd facet of extraversion) and openness in adolescence but then decrease in both of these domains in old age. Agreeableness changed only in old age.'

Looking at the results more closely, the changes reported are statistically significant but not necessarily large. The largest effect they report is for Social Dominance, which increases on average by d=0.41 in the age group 18-22. This can be translated to a change of less than 1 standard-ten score, which is the scale used in Alva's personality test. The findings are nonetheless interesting, and they show that some variation in personality is the rule rather than the exception.

This is evidence to support both the stability and changing nature of personality across the life span. As we age we seem to change in some general ways - as young adults we become more Extraverted, Conscientious, Emotionally Stable and Open, and as we age we become less Extraverted and Open and more Agreeable. These changes are however typically not very large, which indicates a certain level of stability.

Conclusion
Personality is both stable and prone to change. In general, people change over time but not very much. Translating this to Alva's personality test, you can expect your results to differ by 1-2 points if you were to redo the test in a couple of years, while a large change is possible however unlikely.
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​References
​Roberts, B. W., Walton, K. E. & Viechtbauer, W., 2006. Patterns of Mean-Level Change in Personality Traits Across the LifeCourse: A Meta-Analysis of Longitudinal Studies. Psychological Bulletin, 132:1, 1-25.

Vukasovic, T. & Bratko, D., 2015. Heritability of Personality: A Meta-Analysis of Behavior Genetic Studies. Psychological Bulletin, 141:4, 769-785 

Notes
*Vukasovic and Bratko report an average heritability effect of 40 %, which includes other personality models as well.