My mom has an iMac that's almost two years old. It's kind of the family computer and none of them know how to take care of a computer, and it has been steadily slowing down this past year. I cleaned it up for them once and it helped a lot, but they managed to slow it down again pretty quick.
My laptop is a little over 3 years old and it runs just about as well as it always has. The only operating system I've ever had on it is Win7.
So basically I'm pretty sure that if you take proper care of your computer, it will continue to run well regardless of operating system.