Pretty typical sign of wear around here. The granules fall off exposing the fiberglass matt causing UV damage and drying out the asphalt which causes more granules to fall off. It will typically be worse on roof faces with full direct sun exposure.
Be careful if you walk, the ganules will likely give way and you can take a fast trip to the ground!

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002