My girls are 3.5 and have always slept together. For the first 2 years they slept with us (with me in-between) and then we moved them to their own queen mattress on the floor. They love their bed, though they don't always stay in it
Maddie will often come into our bed or onto a little crib mattress we have at the bottom of our bed. Katherine pretty much sleeps like a rock - hence no problems with one waking the other up. Also, I will add that between 1 and 2 (or so) I would often consider having them sleep separately, or at least put them down separately, because they would play with each other and keep each other up. Drove me insane.
But, I never did actually separate them...because 1. I got a little bit more lax in the sense of, if they are going to play, let them, and I can get something else done. 2. We started getting a routine going that they really enjoyed and 3. When they cut out naps, Katherine began to fall asleep really fast
So... in answer to your question, it totally depends on the munchkins, not just in general, but even as they go in and out of stages in development.