Till they decide to have kids, or some privacy. I know having kids is not en vogue like it used to be, but still plenty of folks want and will want them, plus you cant erase biological needs of women that easily.
Multigenerational households make having kids easier. People do it in the rest of the world—the aversion to it is a distinctly Northern European thing.
https://news.ycombinator.com/item?id=37383877
https://ourworldindata.org/fertility-rates/
https://ourworldindata.org/uploads/2014/02/World-population-...
https://news.ycombinator.com/item?id=37300184
> plus you cant erase biological needs of women that easily.
https://news.ycombinator.com/item?id=37325120 ("How South Korea’s birth rate dropped below Japan’s")
https://news.ycombinator.com/item?id=34992144 ("Japan births fall to record low as population crisis deepens")