I would agree that if there is no reason to move them from their cribs, then wait until they're over two years old. It is hard to reason with 19 month olds about why they need to stay in bed. (And they WILL get out of bed, that's a given!)
Our first two were both over two years old when we moved them to their twin beds and it went much better than with #3, who dad moved into her toddler bed at 21 months while he was in charge and I was in bed sick. It's been almost three months and I've just had to give in to the fact that she gets up EVERY nap and bed time after I go out of the room and gets herself books to read. I just have a habit of going in and either laying her back down if she's still awake, or prying books out from all around her if she's fallen asleep.
I second the advice about the gate if you have stairs. My daughter automatically goes to the kitchen to find me when she wakes up, and that takes her past our stairs. However, in the middle of the night I'm in the opposite direction, and I've had bad images of her stumbling down the hall in a sleepy daze and falling right down to the stone floor below.
As others said, begin a very strong routine, and stick to it from day one. Give the same response EVERY time they get out of bed, and you shouldn't have problems for too long.