My boys are 7 and 4 1/2, and they share a room. We have a guest room, so they could each have their own room, but I like having a guest room, and they have never asked to have their own room, so I'm in no hurry to split them up. All in all, I think they like sharing a room. The little one likes everything his big brother likes, so decorating it was not difficult. Within the room, they do each have some of their own space - they have bunk beds, and each one has picked out his own bedding and wall stickers for his bunk. Each has his own toy shelf in the closet, and each has his own dresser. They put their very special toys on display on their own dressers, and are very respectful about not touching things on their brother's dresser.
They fight a lot (normal sibling stuff), but really, I don't think sharing a room is a factor in any of their disputes. Before moving into their room, we had a family bed, so I think they actually really like having someone else in the same room at bedtime.
The room is a little cramped (we do not have a playroom, so their toys are all stored there as well), so space may become an issue for them as they get bigger. I don't have a magic age when we will separate them - I grew up in a large family where we shared rooms out of necessity, so having a plan for separate rooms doesn't seem that urgent to me. As a matter of fact, I was still sharing a room with a younger sister when I met my husband (I was in my early twenties, and had moved back home to save money.) It was sometimes irritating to have to share, but I mostly have positive memories of that time. My husband, on the other hand, never shared a room with his brother and finds the concept of not having your own space really odd. So, our plan is to add a guest room in our basement sometime in the next few years, and give them the option of having separate rooms at that point.