The changes that show in the graph are due to maturation
How is this due to maturation
Studies in the realm of religious belief and age have discovered a correlation between age, life expectancy, and religious beliefs. As individuals grow older, there is an observed tendency for them to discuss religious topics more frequently, potentially leading them towards adopting a more religious orientation.
Moreover, in certain countries, there appears to be a pattern where as individuals mature, there is an inclination toward increased religiosity, marked by heightened faith in a higher power, as evidenced by research findings.