Generally, toddlers will move to a real bed between 2 and 3 years of age. Climbing out of the crib is one sign; another is if your child looks cramped or uncomfortable there. If you're expecting another baby and will need the crib, you might want to ease your toddler into a bed a few months in advance so she won't feel usurped.
To ease the transition:
Get her on board. Let your toddler think it's her idea to move to a big-kid bed. Plant the seed by talking about how cool they are and taking her shopping for fun new sheets. Point out friends or older sibs who sleep in beds. Then ask, "Would you like to sleep in a big-girl bed?" If she resists, wait a week or so, then try again -- she may need to warm up to the idea.
Keep her bedtime routine the same. A little continuity will help her feel secure in her new spot.
Try a bed rail. Placing the bed in a corner of the room and using a rail can make the new bed feel more criblike. It can also help prevent nighttime tumbles.
Note: If your child has other major changes going on (like starting preschool or moving), you may want to wait a month or so before making her leave the crib for good.