How can the study of children, each one unique, be a science?

For starters, every child can exhibit certain traits, based on their upbringing, economic standing, and even the place where they reside, can dictate how they act and how they present themselves. Scientist usually study children to either prove their hypothesis right or wrong. Studying children can give scientist as base guideline as to how children behave and the similarities and differences between them, and their development in their life cycle. The same goes for studying Adolescents, Young adults, and so on and so forth.