Your deck looks like it was well designed to have different zones, but I agree it’s showing its age a little bit. The first project to tackle is sanding and re-staining. This is a big job, definitely one for the pros. The contractor will need to go over the deck with a power washer to remove any loose bits and dirt, let it dry thoroughly, then use a deck-cleaning product to remove mold, mildew and any stubborn stains. After that has dried, the whole deck will need a light sanding; this is also their chance to fix anything that needs repairing. Then after a vacuum or sweep, they’ll stain it with one or two coats. Be wary of choosing the contractor who gives you the cheapest quote or fastest timeline; there’s a lot of work in the preparation of the wood, and that will directly impact the longevity of the stain.