You can deworm them at that age. Yes, Fenbendazole is dosed by weight.
DE won't eliminate worms in your yard. The best thing to do is either get a few samples tested in a couple of months to see if worms are a problem (again) or you can deworm regularly. A lot depends on your environment and soil. Some places that are wet and warm you may find that you have to deworm more frequently than in dry hot or very cold climates.
DE won't eliminate worms in your yard. The best thing to do is either get a few samples tested in a couple of months to see if worms are a problem (again) or you can deworm regularly. A lot depends on your environment and soil. Some places that are wet and warm you may find that you have to deworm more frequently than in dry hot or very cold climates.