My boys began sharing a room when they were about 6 months and 28 months (almost 2 1/2). We delayed moving them into the same room (the baby had been in our room previously) because the younger one was not a good sleeper, and we feared that his persistant and furious crying would wake up his brother. Finally, out of desperation for some sleep ourselves, we tried it, and much to our surprise it was fine. Once in a while the older one woke up, but he went right back to sleep, and usually he slept right through everything. They still share a room (now at 1 1/2 and 3 1/2) and seem to enjoy being together. However, they have always taken naps separately (we put the younger one in a Pack n' Play in our bedroom) because they nap for different lengths of time and are not as likely to go to sleep if they can distract one another.
One thing I would definitely suggest is a crib tent for your baby. We got one we like at onestepahead.com, but there might be others. A crib tent is a mesh tent that zips around the entire crib (so top, bottom, and sides are enclosed), and its advertised purpose is to prevent your child from climbing out and hurting himself. We initially bought it to prevent our then only son from throwing his binkies and toys out of his crib and then crying instead of sleeping because he didn't have them anymore. However, it has also proved invaluable in preventing our toddler from climbing into the crib with his little brother or throwing toys in there. Now that the older one is 3 1/2, he can unzip the tent himself, but the few times he has done it he has gotten into trouble, so it hasn't been a problem lately. A crib tent would certainly buy you a little time when your baby is most delicate and before your older one figures out how to unzip. We could also prevent this problem if we moved our toddler's bed a little farther from the crib so he couldn't reach the zipper!
By the way, having two kids share a room has gone so well for us that we are actually planning to put a third kid in there sometime next year ... our third baby is due in October. We just don't have sufficient space to do anything else.