Old enough to be away from their mother, and probably a couple months after that just to make sure they are big enough. As a rule of thumb, NOT before they are potty trained, although, we've had litters potty trained pretty quickly before.
I've had experiences with one of my Shetland Sheepdogs who's now 2yrs old, but when we were potty training her, I let her sleep on the bed and she was usually pretty good, but she did pee in the bed a few times (yuck!!!!!!!
) so, atleast not until they are potty trained.
I have two males, they usually only hop on the bed if it's storming outside (they get scared), or we let them up for attention, they are 7 and 8yrs old. The female, we no longer let sleep in the bed, she goes in a kennel at night now, along with her playmate, another female. Only because the youngest female chews everything up if you leave her out (she's 2, so I hope she quits soon!)...and the older 2yr old stays with her just to keep her company.