It's easy to think a sturdy wood deck should be relatively impervious to the elements, for a few years, at least. The truth is, though, that decks take a lot of punishment over even a single season. The flat surface of a deck responds to the elements somewhat differently from, say, a wooden wall. Water dwells on the wood longer, and sunlight can be more concentrated and damaging because there's no overhead protection. Foot and pet traffic, as well as moisture problems caused by landscape plants and other objects obstructing good air flow can make your deck age prematurely, too.
Over time, nails can partially work their way out of deck floorboards and cause injuries. Boards may warp or crack, dirt and dead leaves can accumulate between the floor boards and bird droppings, dirt and pollution can stain and discolor the finish. These things don't happen all at once, but keeping a sharp eye out for evidence of insect activity, mildew growth, loose treads on the stairs, ponding and wobbly railings can do a lot to help you understand how your deck is aging and develop a strategy for effective seasonal maintenance.