I pull weaners more on look than date. When they get a pointy nose, usually between 3-4 weeks, they are ready to be pulled. If you are raising them for more than just food, ie to replace a breeder, leave them in for an extra week to gain some size.
Most of mine are pulled between 3 and 4 weeks old, but like I said, you kind of get a sense of just looking at them if they are ready to go. If left in with mommas, they will continue to nurse, possibly starving out the next brood...