When Microsoft stops supporting an operating system, such as XP Service Pack 3, on April 8, 2014.
When the new software you just have to have won’t work well on your old computer.
When you are learning a new computer skill, like music editing, on that old computer that just barely handles email.
When you can bake a cake while the computer is starting up.
When new hardware, like printers and cameras, don’t come with drivers for your older operating system and computer, such as might happen if you have OS X 10.4.11 or any PowerPC Mac.
When your workplace pays for one.
When you cannot switch between four open programs without a delay.
When your tower is over five years old and you have to replace most any part, or your laptop is over three years and you have to replace the screen.
When you move into a smaller place and the all-in-one computer or laptop fit better in your camper.
When you count up the hours spent waiting and looking at the screen, multiply by the current minimum wage, and find that 20 wasted minutes each day (assuming a five day work week) comes out to over $600/yr. And, there is your new computer.
When you win the lottery.
What new computer? The iPad or other tablet may work just as well for your needs.