Well, I don't think it is really a matter of how old she is, but that you have a rooster to fertilize the eggs. I would suggest that you crack open a few eggs and check for fertility. There are a couple of threads on this website about that. I will look for one and post a link for you. Off hand, I know there is one that SpeckledHen started that shows pictures. Again, as long as your hen is laying and your rooster is doing his job you should be able to put eggs in the bator. Hope this helps!
You need to wait until the eggs are about the standard size for the breed of chicken - otherwise, the embryos cant develop properly! Hope this helps!