The 2016 Toyota Prius is about as different as a new model can get, with radical new styling, a greater emphasis on performance, and even greater efficiency than before. When it arrives in showrooms, the fourth-generation Prius should make a big impact.
The easy answer to what happens to car engines as they age is that they wear out. The much more complicated answer is exactly what begins to wear in an engine and when it does. Here's more information from Gray-Daniels Toyota.