Build additional houses and either the singleton elderlies will move in or those younger couples ready to start families.

A "Retirement Home" would be nice to clear up some homespace, but I don't know if you can force them to actually move anywhere due to age. Even Students just basically "join" the school as a kind of timed profession, I think. But, it does have an age-related hook to it - 10 to 17.

Maybe force a "sick" trait at a certain age and try to tie it to that? /dunno, not a Banished modder

Can't be done I'm afraid. There's no way to force people to move out of houses other than marking them up for destruction.
All you can do is encourage people to move by building new houses. The "comunal housing" type of things like inns will take in homeless people, but it's not a place that's marked as permanent housing, as soon as you build a new house, people in there will move out, often faster than adult children living with their parents in normal houses, so a boarding house isn't an option either.