There is only one kind of housing that may lawfully exclude (not allow) children. That is housing that is for people who are 62 years old or older. Every person who lives in this housing must be at least 62 years old. That means that a 60 year old wife cannot live with her 64 year old husband in this housing.
It is easier to set a rule that housing is for people 62 years old or older when it opens. It is hard to change from mixed-age housing to housing for seniors over age 62 without discriminating against people based on age or family status.
Some housing is set aside for people who are aged 55 or older. This housing requires a certain amount of the residents to be at least 55 years old. As long as that amount is met, children and people younger than 55 can live in this kind of housing.