Windows requires more and more hardware power as its version number increases (95, 98, 2000, Me, XP, etc.). So if you want to keep running Windows, you need to constantly buy new hardware. But I can't see any good reason for so fast an evolution. Of course, many people need a lot of computer power and new hardware and technologies are really helping them. But for most users, who surf the web, read and write emails, write text files and slides, what's the point of buying a new computer every 2, 3 or 4 years, apart from letting computer vendors earn more money? What is exactly the profound reason why your computer can't do any more of what it did perfectly well 5 years ago?
Linux runs perfectly well on older hardware, on which Windows 7 would probably even refuse to install, or leave you waiting for 20 seconds after each click. Of course, Linux won't make a race-winner out of your 12-year old computer, but it will run very well on it and allow you to perform usual tasks (surfing the web, writing documents, etc.) just fine. The very computer that delivers this page to you is not very young and runs Linux: if you can read this, then it is up and running (and if the website loads slowly, blame my Internet connection only).