I'm running on a system that's roughly seven years old. It was pretty good when I had a relative build it, but it wasn't top of the line even back then.
I howls, grunts, clanks, refuses to burn anything CD-DVD-wise- but it plays the game fine, and I certainly don't feel like cannon-fodder. Sometimes it smells funny (unless that's me?).
It's high on my wanna-replace list, but low on my have-money-to-toss-at-it list. It's also tried to keel over and just plain die on me a few times, but I've been able to toss some duct-tape and band-aids in there, and she's still breathin'.
The basics-
2.20 gigahertz AMD Athlon XP (Processor)
1024 Megabytes Installed Memory
Board: nVidia-nForce
Bus Clock: 200 megahertz
NVIDIA GeForce FX 5700LE (Omega 1.6177)
Creative SB Audigy
Now, last time it threw a tantrum, I ended up taking my back-up HD and made it into my Primary HD. When I did that I partitioned it so I could turn off all the bells and whistles in the partitioned area, which is only used for playing AH. I don't think I could play if I didn't reduce the amount of processes sapping my resources. Before I did that (the partition), I went in and created a seperate Start-Up Profile doing basically the same thing, with basically the same results game-wise. That worked fine for me for about 5 years.
Maybe your system isn't too terribly bad- maybe that's not really the issue... If it was an inexpensive ystem 6 years ago, it may be. I don't think we can judge it by age alone.
I'm getting about 70 Frames Per Second just flying around.