That would be in the pre-Renaissance period.
His ideas on development of the childhood as an idea and a concept was presented in the book Centuries of Childhood. He argued that childhood as we know it today, and as a recognizable concept and as a concept at all did not exist before 15th century. His conclusions and ideas were inspired by the paintings of children up to this period where children were literary painted as smaller versions of adults without any noticeably different facial expressions and features.
