we used them from birth until 6 months. Oliver was 8lb7 but was a very skinny and long 8lb7 so the tabs crossed until he was about 4m and the only reason we stopped using them was that he was outgrowing the rise. I bet I could easily put one on him still now
The one big tip I would give would be to remove the silly bulky mf insert that they come with. You dont need them to start with, and if you do need more absorbancy then a folded size 1 LL bamboo booster fits perfectly in them and adds a lot of absorbancy for very very little bulk. They were by far my fave nb nappies, and we used diddys, wahm, LL's, and prefolds too from birth.