I get this question a lot for various reasons: smaller spaces, wanting a playroom, vacationing, or just parents wanting a guest room or office. The simple answer: any age, but I recommend waiting until the youngest is at LEAST 2.5 years old (sleep regressions before then happen almost every 6 months, yikes.)
If you decide to go for it, here are some great tips I can give to make this process go smoothly for those parents out there dreading the idea (or those lucky enough to be expecting twins!) and still want their babies, toddlers, twins, or older kids to sleep through the night!
#1: Always put the youngest child to bed first. Generally your youngest will naturally have an earlier bedtime due to their biological sleep needs, so begin there. I always recommend that a child's bedtime routine end in their room (reading a story, etc.). While you are putting down your youngest, hopefully Dad is home to play with your oldest or he/she can understand that you'll be absent for a few minutes (a cartoon is my last resort!) and doesn't come barging in the room. Afterwards, make sure to carve a little downtime for your older child as well.
#2: When sleep training one child but not the other, separate. I see this a lot with twins- one is a great sleeper while the other has some issues getting (and staying) asleep. For this situation I recommend that you temporarily separate the kids while you sleep train so the "sound sleeper" isn't bothered during the process. Once you feel as though the kids are ready to be in the same room, move them back together.
#3: Manage your expectations. Sharing a room is EXCITING!! Even with the best sleepers, prepare yourself for a few nights of giggles and play. Toddlers love a good reason to combat sleep, right? If you need to step in if things get too rowdy, go ahead, but explain to the older one that this isn't a time to encourage hide-and-go-seek.
#4: Will baby wake up my older child? Yes, probably. And I'm not talking about small noises here but the "super-loud" crying type of noise. My advice here would be to wait until you really know your baby is in need of assistance (depending on age) and first assure the eldest child that everything is alright and "to go back to sleep"- and THEN tend to the younger upset child. I've taken my baby out in the hallway on vacations (while sharing a hotel room) to calm her down and tip toe back in to lay her down after she's calmed down, minding the sleeping toddler whom had already fallen back asleep.
#5: Sleeping in the same bed? Sometimes. It's important while I advise my clients to keep the crib as long as possible for young babies & toddlers, if you would like your 18+ month old to share a bed with their older brother/sister, that's ok! If your oldest is fine with the idea, I've found that generally children love the idea of sleeping together in one bed at night. Some parents even claim this decreases sibling rivalry and fighting. They may also bed-hop if you have two beds in the room- and eventually with age will settle in their own respectable beds.