Determining Age of Windows Computer

systeminfo | findstr /C:"Install Date"

and looking for the BIOS version will also give you a clue:

systeminfo | findstr /C:"BIOS Version"

• If you have access to the physical internals of the computer, i.e. if you can open the case, and if you are willing to go through the trouble, you might find some date labels on some components (like the hard drive).
  I've used this method several times successfully, but it depends on where the computer was bought (small company, big vendor like Dell, internet, ...) and if they have a habit of sticking date labels to certain components.

• If it is a computer from a notable manufacturer (Dell, HP, Apple, ...) you might try to look for a sticker with a service tag, and enter that service tag somewhere on the company's website. Some vendors show the purchase date.

Everest can show this and lots of other info about your PC.

If you mean: "When was Windows installed on this PC?" then for manually installed Windows you could look at the creation date of the Windows directory. This wouldn't work if the PC was cloned from a from a pre-made system image, such as a PC from large OEMs/corporate PCs. But if you know that you created your user account around the time the computer was first booted from the image then look at the creation date of your user directory. I'm assuming that you're using NTFS & not FAT-32 as the disk format :)