What is a word for: An area of someones life that explains why they are the way that they are?

Question

Examples:

Jimmy lived with his single mom and became a really big bully. Living with his single mom is the area of his life that explains why he is a bully.

Sandra was called fat when she was 13 years old, and she became anorexic. Sandra being called fat when she was 13 years old is the area of her life that explains why she became anorexic.

Leo's daughter got in a death-defying car wreck with in 18-wheeler and now feels uneasy around them. Leo's daughter getting in a death-defying car wreck is an area of his life that explains why he feels uneasy around 18-wheelers.

Answer 1

Such a period/experience/influence can be termed as formative and in some cases life-changing.

ODO:

formative ADJECTIVE

1 Serving to form something, especially having a profound influence on a person's development.

‘My father poured vast amounts of love and energy into me during my most formative years.’
‘In both cases, learning to read was one of the great formative experiences of their lives.’
‘His mother was a formative influence, since he did not attend school.’

life-changing ADJECTIVE

Having such a strong effect that it changes one's life.

‘the whole trip was a life-changing experience’
‘You might think, for instance, that winning the lottery would be a life-changing event that would brighten your outlook for years to come.’