Answer: Fall
Explanation: The first stanza (that is the first four lines or quatrain connects old age to early Winter. In such quatrain, Shakespeare's sonnet evokes the natural decay of all things through a series of images, all connected to a tree. A tree with yeallow leaves or a few of them is a common image during late Fall while a tree with no leaves at all is mostly seen in Winter. This idea is reinforced by the phrase "shake against the cold," which evokes the arrival of early Winter.