My G4 Sawtooth which I purchased in 2000 lasted me until 2006 when I purchased a Mac Pro 2.66GHz (6 years), given, I had to upgrade the processor to a 1GHz G4 and I had 1.25GB of RAM installed, and upgraded the hard drive to a 20GB drive.
So ten year may be stretching it. You could still use a G4 even today with 10.5.8 and still be able to surf the web and run Photoshop and iTunes, you see....but the Intel Macs all basically made the G4s obsolete, and the G5s are pretty obsolete by now too, and they are almost ten years old. A five-year-old Intel Mac is not obsolete now, but Apple will eventually drop support for anything but EFI64 and then it will only be Core2Duo or better that will be supported anymore. We are then talking 2007 or later (4 years old or newer)
It might be 2017 or 2018 before some new EFI standard comes around, EFI128, and then all support for EFI64 Macs will eventually be dropped. But we are now speaking Post-Ivy Bridge Haswell and Rockwell processors here...that is probably at least 5 or 6 years off.