I don't think there is a good age, its when you can trust your dog alone in the house. Ryker and Ozzy only stayed in their crates at nite time till they stayed put. During the day I started by making sure they had sufficient toys etc to keep them occupied then would pretend I was leaving without a fuss and stay outside for five minutes and come in without a fuss again and extend the time till they were fine, this was like over a week just to get them used to it. Close all doors and put anything thats worth anything up and out of reach. It all worked out, Ryker never chewed anything but Ozzy cost us a few bucks

You can always keep them in a room, like close off the kitchen or a spare room. Some dogs do fine others will tear everything apart. I think if you work on the leaving them for little amounts of time and they know your going to come back it should work out.
Is she out of the crate at night?