Until they're grown, if they like. Heck, I have friends from the UW who rent an apartment with their siblings in order to save on expensive dorms, but still be 'out on their own'. Studio apts...they don't share a bed, but they're definitely sharing space.
I also have friends (mostly male, and mostly I met them in the USMC) who shared rooms with their sisters or their sisters and brothers until leaving home. (Actually I knew a few who their entire FAMILIES shared one room...because that was all they had. Puts a bit of a spin on things, doesn't it? ) Grand people in general, whether they shared rooms with just siblings or the whole family, more caring and conscientious of other's feelings/ or need for privacy/or need for (non-sexual) consoling/etc. than most people I know. I also know many many others who split rooms at puberty...they tend to be as shallow and self centered as the rest of us...so maybe it's the going through a self centered phase with nowhere private to storm off to that does it? Or____? Or____? Who knows.
My grandparents & great grandparents also all shared a room with the opposite sexed siblings (they just didn't share beds...there was the girls' bed and the boys' bed...up in the loft). Frontier folk, and early settlers. Their parents? Some had their own wing (day nursery, night nursery & servants), others were crammed into the irish slum crackerboxes...but I don't know either of their sleeping arrangements. Although, just thinking of peter pan...wendy shares the night nursery with michael & john, doesn't she? That's right. That's why they run away with peter, because she didn't want to have to grow up and have a room of her own away from her brothers.
While it makes a heckuva lotta sense for foster children to have their own rooms (for their own, as well as the other children's protection...as well as many many other reasons)...I don't personally see a problem with children growing up close with their siblings. They're SLEEPING, for heavens sakes. Or playing just like they'd play in the daytime. If you don't keep them separated in the daytime, why keep them separate at night? If someone is having sexual thoughts about a brother and sister that's THEIR freakishness...ewww....not the children's.
PS...Just talked with 2 lawyer friends. They know of no jurisdiction in the US where it is illegal for true siblings of the opposite sex to share a room, at any age...nor is it likely that any law or precedence would ever pass, as it is not illegal to be poor, and that there are millions of poor people who cannot afford more than a 1 bedroom, much less a 2+ bedroom residence. Not to mention millions more of middle and upper class people who believe that the govt has no business butting in on healthy family dynamics. It's only in fostercare that there are laws about opposite sex nonsiblings sharing a room.