i can run XP on a 9 year old computer
I would certainly hope that you could run an 8 year old OS on a 9 year old computer!
can you run leo on a 9 year old mac? (this i think would be a G3ish computer)
The G3's were superseded with G4's circa 2000-2001.
The official minimum requirement from Apple for Leopard is a 867MHz or faster G4, which means that the G4 'Quicksilver' (introduced 2001.07.18) probably qualifies, which is only an 8 year old machine.
To turn the tables back to Windows, this would be like asking XP to run on an 8 year old PC, which would be 2001-8 = 1993. And a 1993 vintage PC would be approx a 66MHz Pentium, which falls well short of XP's official minimum of a 233 MHz Intel Pentium/Celeron family (with 300MHz recommended).
The exercise could be repeated for Vista: a 1999 PC (550-733 MHz Pentium III), versus an official minimum of 1 GHz (and support for DirectX 9 graphics and 32 MB of graphics memory would probably be a challenge too).
For Win7 to run on a 2002 PC (1.6-2.4GHz Pentium 4) will probably work, "but...", plus this is probably illustrating more so that the rate of change for computer horsepower has flattened out.
-hh