I'm always a fan of waiting as long as humanly possible...I think it's easier once they're older and are better able to understand what's expected of them. That said, we moved Tyler to a big bed at 29 mo because of a baby on the way. We would have waited longer if we could but he did fine with the transition. A couple of ugly weeks but we all survived.
If the timing's right, and you're ready for a few potentially exhausting weeks, go for it. If you decide to do it, you might move the crib out completely. You can even have her help you take it down so she understands that it's gone. We have a trundle bed too, we didn't even have to use guard rails on the new bed...just pulled the lower mattress out so he had a soft place to land if he rolled out. It didn't take him long to figure out how to stay in the bed.
