DSD and DS shared a room (by choice) until the ages of 10/6 respectively when she decided she wanted her own space.
Thankfully not an issue as we had a 4 bedroom house at the time, so she chose one of the former guest bedrooms as her own and DS stayed in their former room - we just decorated them both to suit the new arrangements.
Children having their own rooms is obviously ideal, but I don't think it's a big deal until they get to pre-teens (assuming different sexes).
Even at 10, I think DSD could have shared for another year, 18 months or so at a push but obviously no reason for her not to have her own room in our case.
In the case of same sex siblings then it's nice not to have to share but frankly it's a bit of a first world problem really.
That said, I can't fathom families who have more children than they can afford to accommodate appropriately.
If you can only afford a 2 bedroom property then being concerned about 3 children sharing a room in a triple bunk bed, 2 different sex children sharing or similar long term makes me put on my judgy pants if I'm honest...(excepting circumstances like finding out you're expecting twins/triplets).