Hi A.,
My two oldest boys started sharing a room when they were almost 3 and 16 months. We used that transition to move my younger son to a toddler bed. It made his transition to toddler bed easy, but the sharing a room part was pretty much a disaster. They thoroughly enjoyed it, but that's because they made naptime and bedtime one big party. They weren't getting near the amount of sleep they had prior to sharing a room and we were seeing the effects of too little sleep in their behavior. We tried several suggestions I had read about. Like, not having toys in the room, making it as dark as possible, childproofing the room, etc, but nothing worked and after a few weeks I decided to just have them go to bed at separate times (now, if I had not been 7 months pregnant with twins, I might have been able to stick with trying longer, but at that point I just wanted to sleep, lol). That worked at bedtime, but not naptime. Fortunately, we have a spare room available most of the time (my step-son's room) and that is where my oldest falls asleep at night and sleep for his whole nap. If that weren't an option, I would have just kept working at getting them to fall asleep at different times. Now that it's been almost 6 months, we can sometimes put them to bed at night at the same time, but it still takes them a very long time to actually fall asleep. So, in my novel here, I guess I'm saying, I would do it as early as possible and just go with the flow. It's a transition and it will take time.
Good luck! Let us know how it goes!
C.