1. As inevitable the physical decline is, do your best to stave it off. It is easier to stay in shape than to get in shape, and the older you are, the harder it will become.
2. Some badminton qualities are less affected by age, and others not at all. It is likely still possible to improve consistency, footwork efficiency, technique. Maybe add deceptions to your arsenal. Another factor of time, and this may or may not apply to you, is that the older you are, the more likely it is that you have won whatever you wanted to win, or you have proven yourself to others, and to yourself. This takes the burden of expectation off the shoulders. Younger players are usually more eager to win, and have psychologically more at stake. This results in the trend that older players are likely more mentally resilient and less likely to psychologically falter during a game.
3. You can play in a way that takes the match away from fitness. Ahsan and Setiawan try to keep rallies short, relying on superior shot quality, their aggressive netplay, their ability to play difficult shots under pressure, and mental resilience. At the club level it's a bit of a stereotype to have two older (often heavier) players that stand still and just defend everything. Often they win points by counterattacking. Bonus points if they have deceptive skills as well. It's far less physically demanding to defend than to attack, so as long as the defense is good enough it's a viable strategy.
In singles it will be more difficult, but still possible. To balance the scales against a younger fitter opponent, the goal is to win your points as cheaply as possible and to make sure the opponent has to work hard for their points. If you can play deceptively without making mistakes, it is a great tool to win cheap points, or at least to tire the opponent more. Playing into the opponent's weakness is another way to take the game away from a fitness-competition. At lower levels, playing the opponent's backcourt backhand are simply free points, but even at higher and higher levels, the backhand is still a weaker and less powerful side for most players, and playing it relentlessly can be a way to put pressure on the opponent while making your own court smaller.
Badminton is a complex game where many factors play a role in the outcome of the game. Fitness is only a component, that while important, can be minimized by playing in ways where the players' fitness matters less. What do you consider your strengths? Try to play in a way where those strengths have more influence on the game.