I was born in 1994. When I was 9, my parents decided to buy their first computer. As advised by a friend, they bought a Mac. The seller, somehow, convinced them to go for the highest range model. "It will last a long time" he said. It was a PowerBook G4 15". It cost 3000€. Huge purchase. I still remember to this day my dad carrying the huge box (these were quiet bulky by then) along the 500 meters which separated my house from the store. It was 2003. Facebook didn't exist. Twitter didn't exist. Even Google was unknown to us.
Some 12 years later, here I am, writing this Quora answer. On my first computer. Our first computer. Can I run 2015 games on it ? Probably not. Can I get the latest OS ? No. Would I say it's obsolete? No.
I manage photos, musics, films with it. I browse the web (a lot). I download things (a lot). I write programs on it (sometimes). I write docs on it. I basically use it in the same way as 90% of people do with their computers. And whatever I do, it does it. Sure, the battery ain't working anymore. But, seriously, who needs a battery? It's my computer, I won't let it go. Never.
As for the 2015 retina MacBook Pro sitting next to it, which boasts so much more horsepower, so much ram, so much PCIe SSD compared to the latter, I don't even envision it becoming obsolete.