Which stage of the human life course became a clear and distinct stage only in modern societies?

The answer is "childhood".

Childhood is the age length going from birth to adolescence. According to Piaget's theory of cognitive development, childhood comprises of two phases: preoperational stage and concrete operational stage. In developmental psychology, childhood is partitioned up into the formative phases of toddlerhood (figuring out how to walk), early childhood (play age), middle childhood (school age), and adolescence. Different childhood elements could influence a man's state of mind formation.