My situation is that I have 3 children - girl of 9 (nearly 10), boy of 5 and girl of 11 months.
I have a third floor which is a (proper) loft extension but this is my office/playroom/used to be husband's bedroom before he moved out.
My son has his own room and my two daughters are in together. When my eldest daughter wants her own space, I will let her sleep in the loft (integral part of the house) but at the moment she is happy to share, especially as baby is not crawling/walking (another story!) and is not into her things. Although the loft is the playroom, I find that my DD1 does prefer to play with her things in her bedroom but I keep all the toys they are not playing with in the loft.
Regarding sleeping together - I am not sure if this is relevant or not, but when you go on holiday and some hotels make/let children bed-share, this is only if their combined age is less than 11 but obviously this is not relevant as they are not actually bed sharing, you are only talking about sharing a bedroom. I think the only time a problem might set in, is when the eldest starts puberty and starts going through what all pubetic children go through and then they want their own space.
A friend of mine who has a council house has 3 bedrooms and has a boy of 13 (own room) and twin boys (age 7) and a girl (5) who share a room - boys in bunk beds, girl in single bed. She wants a larger house but the council don't seem too concerned that two boys (aged 7) are sharing with a girl (aged 5).