Age is just a number, but not for decks. Decks have a hard and cold use by date. When that date passes there is little you can do to prevent the inevitable. The sun, the wind the rain, and all the elements of nature combine to kill your deck as quickly as possible. If your deck is made of raw untreated wood then you can count on having it for less than ten years before needing to have it either fixed, replaced, or torn down. Now dot go jumping to pressure-treated wood, that stuff is poison. The chemicals used to make it are so dangerous that recycling it can poison the groundwater. It should be avoided at all costs. Natural raw wood can be treated safely with other chemicals. A bio-safe varnish that protects against UV radiation while sealing the wood off from moisture damage will prolong the life of your deck well beyond its natural potential. Taking precautions when it is first built is key. Use brackets to adhere the larger pieces of wood together and make sure to varnish every side of every piece of wood you are going to use to make the deck. Cut first, varnish after. When you drill a hole to put in a screw or bolt, bolts are better and will last longer, make sure to paint in some varnish into the hole and seal it with the bolt. This will make the intersection of the wood and metal waterproof and will drastically reduce the chances of wood rot.
If you have a deck already it would not hurt to give it a clear coat of UV-resistant varnish but the underbelly is the main workings of the deck and without that being structurally sound the deck boards on their mean very little. One of the fastest ways rot can hit wood is when it is buried in the ground. The water collects and sits and infuses the wood with bacteria that want to eat it. This bacteria can only eat untreated raw wood. So if you treat every side of every piece along with the drill holes nature will not be able to infect your deck with rot for a least the first few years. Keeping up with it and inspecting the underside can go a long way to keeping the structure stable for the long run. Make sure to use strong wood. Do not go for the cheap stuff because it will eventually break from overuse. Oak, cherry, and redwoods are the favorites and the best options for a long-lasting deck. While we all wish our deck could be made of Ebony, a wood nearly as hard as steel and with no textural grain. But to build even a small box from Ebony is a very expensive process. Focus on cherry and oak as redwood is greatly endangered around the world.
Now, nothing you do will ever be enough. Your deck could have to be made of stone to last any longer and even stone needs repairs after a while. The best option is an oak deck treated on every side and in every drilled hole with a UV resistant varnish and to make sure it is all very well waterproofed and that deck will last you decades.