What forces impacted the life of John F. Kennedy from the viewpoint of developmental psychology?
Your question is a bit vague as developmental psychology covers infancy through old age and dying, and though Kennedy died, his death wasn't of the sort that developmental concerns would be applicable.
We can apply Baltes' theory however and posit some sort of direction for your question to take toward an answer. Baltes posits that development psychology applies to an adult, such as John F. Kennedy, in that (1) expertise in specialized cognitive fields develops while other areas of cognitive function fade into disuse and that (2) cognitive development is contextual, which is that it depends upon interaction with the given culture, history and normative elements.
This might be illustrated in Kennedy by pointing to his growing expertise in politics and his declining practice of military skills.