What does 'formal institution' mean?

for institution has various meanings...

If we want to describe our society in terms of age, we may come up with four age groups--childhood, adolescence, maturity, and old age. We take it for granted that people of different ages behave differently. For example, we feel that a man in his thirties should act his age and not behave like an adolescent or an old man. Equally, we expect that, as they go through life, people of the same age will in some ways understand each other better than people of different ages. All this is part of expected ways of behaving in our social life, but it is not something that we can apply in formal institutions governed by hard-and-fast rules.