A deck in full sun in the Central Valley might go thirty years. The same deck under redwoods in Felton or Boulder Creek can start rotting in ten. The difference is moisture: shade and fog mean boards and joists stay damp for much of the year, and damp wood is where rot starts.
The first places to check are where water sits. Look at board ends, the tops of joists under the deck, posts where they meet the footings, and the ledger board where the deck attaches to the house. Push a screwdriver into the wood. If it sinks in easily, that section needs replacing.
Airflow and spacing make a real difference. Boards laid too tight trap leaves and water, and a deck built close to the ground with no ventilation never dries out. When we rebuild, we space boards properly, use joist tape on the framing, and keep debris from collecting underneath.
Regular upkeep helps too. Sweep off needles and leaves, keep planters off the boards, and reseal or restain every couple of years. Catching a soft board early is a small repair. Waiting until the joists fail is a rebuild.