A sapling is a young tree — past a tiny seed-plant, not yet a thick old trunk. You can often still bend it. Gardeners and forest people use the word when they plant, protect, or move young trees. It is a gentle, slightly countryside word. People sometimes stretch it to a young person, a sapling of a boy, but that is poetic, not everyday talk. The picture to keep is a thin young tree, still full of life.