I don't see 24 on the Boise NF Travel Plan, and there is no ID state highway numbered 24 in that area.
Highway 21 makes a long, twisty decent into Lowman. It's not to be missed if you're in Stanley already. At Lowman Highway 21 turns south to Idaho City, and Highway 17 brings you from Lowman to Banks. The ride along the South Fork of the Payette is tremndous. Pine Flats camground west of Lowman is a winner and there are many others.