To be grateful for someone or their actions, one must be able to take that person’s perspective and understand their kind intentionality. This perspective-taking ability, or theory of mind, begins to develop around the ages of three to five; so children younger than this are lacking key building blocks for gratitude.
After this, however, gratitude may not only grow but become more readily expressed, as language skills develop. This tends to taper off, though, during the adolescent years. Gratitude involves acknowledging a sense of dependency, and this goes against the teenage inclination to assert independence. Adulthood then shifts the gratitude slope in a new direction.
Research has suggested that as adults age, their brains become less reactive to negative information while equally or even more reactive to positive information. This translates to a generally more positive outlook, which facilitates more gratitude. Psychologists have also noted that accumulated life experience along with a shortened sense of time seem to factor into the increase in gratitude that tends to come with aging.
Photo credit: anjanettew vis flickr cc