The best time to switch a child from a crib to a bed is around 3. Before this age children don't really have the cognitive ability or the behavioral control to understand that they have to stay in the bed. Although some children do transition around age 2 and do just fine, in general switching a child too early can turn a good sleeper into a waking nightmare. If your kid is climbing out of her crib, experts suggest getting a crib tent instead of relying on a bed to solve the problem. Chances are if your child gets out of her crib, she'll hop up out of a bed as well. If you're expecting another child and would like to avoid buying a second crib (and who could blame you) consider keeping your new baby in a bassinette at first, which should buy your older child a little more time in her crib.
Once you move your child to her big-girl bed, don't be afraid to back-peddle a little if the transition doesn't go so smoothly. It's totally fine to let her move back to the crib for a few more weeks. Keep the bed in her room, however (beautifully made up for her with bedding and stuffed animals she helps pick out) and eventually she'll want to sleep in it. It could be that she just wasn't ready -- and rushing her is only a recipe for disaster.