My PC was bought in December 2012, so that makes it about 4 years old. The only upgrades its had is a new GPU which I added recently (AMD RX470) and a new PSU that was added after the inital PSU got fried
Bought mine in July of 2010, only upgrade is that I added an extra hard drive or two. Although, I've replaced the standard keyboard with a Roccat in 2011, and have gone through 3 mice in the time. Damned Cookie Clicker.
The oldest parts of my PC I bought in April 2011 (my case, PSU, hard drive or 2, screen), the rest I have upgraded since then, with the most recent upgrade being the CPU, MOBO, RAM and CPU Cooler just several months ago.
I bought mine in January so it's not even a year old. The one before that I bought in 2010 and my wife currently uses it. I just upgraded the video card and the RAM.
I recently upgraded most of my PC in December last year.
But some of the oldest components are my 22" LCD Samsung (1680x1050) at 8 years and still going strong.
Keyboard - Razer Lycosa a bit worn but otherwise fine, about 7 years
Raidmax Aztec case - 7 years (could do with an new case, mostly had to replace fans though)
Geez, this is a really difficult question to answer because I started building my PC in 2010 and I've been adding, upgrading and changing things on it so much that the PC I have today is not the same thing I built 6 years ago.
Case in point, what I posted here has already had two things changed in it. I upgraded my keyboard to a Corsair K70 and my GPU to an R9 Fury X. I think the oldest component in my system is either my Blu-Ray writer or my Asus sound card, otherwise everything else is new or has been replaced in the last 3 years.
Different parts have different ages in my PC. My case is very old, Coolermaster Stacker, think my brother originally bought it somewhere around 2005. My CPU and RAM I bought secondhand from my manager and I think they are about 3 or 4 years old. The motherboard I got as a hand me down when one of my brother's friends upgraded, I reckon it's also 3 or 4 years old now. My GPU is 2 weeks old.
Mine is still in the womb...lol It's a month or two away from completion. I've received the CPU and AiO as well as some of the lighting and am waiting for delivery of the rest of the parts
I got my first PC that I bought with my own money back in mid-2007. Almost ten years later, and in theory it's still the same PC, but not a single part remains from the original build. Every component has been upgraded at least once, some more than that.
I find that swapping out one or two bits at a time is much better than trying to keep the same PC for five years and then upgrading everything at once.