Engines are machines, so as they age they're going to break and need repair or just become less and less efficient until you're once-beloved car just isn't as fun to drive anymore.
That being said, there are a few signs you can look out for and some maintenance (some easier than others) that you can do to help make sure your engine is in peak working condition throughout its life.
Jason Fenske posted a new video on his channel, Engineering Explained, to, well, provide an engineer's level overview of why your motor just isn't as powerful as it used to be. The top 10 reasons he spelled out ranged from dirty spark plugs to engine knocks. Obviously, at the end of the day, regular scheduled maintenance is the phrase to remember, so don't be cheap and treat your car to keep it in peak shape.