If you do it by the book so to speak they are too old but I have read on BYC of a hen taking older chicks so if you do it, sneak them under her and hang around to see what she does when they come out from under her. They may be killed when they do this by a peck to the head/body so you have to be there to rescue them immediately. Usually it is good to introduce them at night but you may not be there when they wake up and come out from under her. So I would not do it at night- I would do it when you can be there.
I have had broodies reject day old chicks. If she shows any animosity to them at all when they emerge get them out of there. If they aren't emerging, make one emerge after a few minutes of sitting and watching. A good mama will never peck at her chicks.