I was taking apart and putting computers back together at around age ten. I first started tinkering on an old compaq with a K6-II CPU and compaq OEM motherboard (naturally) using an SiS chipset. To overclock it, I had to use DIP switches on the motherboard. The hard drive had a whine that should have been unbearable. But it was all mine, so I didn't care much. By the time I was in middle school, I had also taken apart and re-assembled a fair share of computers and laptops. Usually fixing whatever issues needed fixing... fortunately.
The first computer I ever built from the ground up was bought mostly as a TigerDirect bundle. I was 13 at the time. It used a Sempron 2500+, 512MB of DDR (don't remember the speed, probably 400MHz) and a GeForce 4, which was later upgraded to a FX 5600 (not the best decision), and then to a 6800XT. The motherboard was a Chaintech 7NJL6, which used an nForce 2 chipset. For storage, I used a single Seagate IDE 60GB HDD.
When first putting it together I cracked the CPU die when clamping down the heatsink (Socket 462). It was a quiet, yet sickening crunch. Ugh... Surprisingly, it still booted up to Windows XP just fine, but doing anything more complicated than launching Wordpad resulted in a flurry of errors and BSODs. Thankfully, I was able to RMA the CPU without any fuss, and the replacement was installed without issue.
That machine had a fairly long useful life. I used it throughout high school, and a little bit beyond that before finally building an entirely new Phenom II based system, which I still make use of to this day.