What are ways to figure out how old my computer is?
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There is no definite way know when was the first time someone booted the machine , but what you can do is download CPU-Z and check the motherboard model no.
Then find its release date and if you can get the date for when the manufacturer stopped making the model you have a date range for the machine .
Usually people don't really change the motherboard so this is your best bet .
In case you are using a branded machine you can always contact the manufacturer with the machine's serial number and they might provide you the manufacturing date.
Are you running Windows? Go to command prompt and enter "systeminfo | more". Then look for the line items that have Original Install Date for when the OS was installed, system manufacturer, system model, processor type, BIOS version, etc. Depending on exactly why you want to know how old your computer is, the above information should be able to provide it or a sufficient enough answer by getting further information from the manufacturer (e.g. processor was initially released in Month/Year).
Depending on your BIOS, you may have a date that it was installed on the system, this is essentially the "birth date" of your computer. I think most IBM clone machines (ie. non-mac) use Phoenix BIOS now, the date is the very last line on the POST screen.
If you are using Windows, this will also work (it says Win2k, but it works on XP): HOW TO: Determine BIOS Date on a Computer Running Windows 2000
If your computer was purchased from one of the major PC manufacturers (ie: Dell) then you can type in your service tag/serial number to find out when the computer was purchased.
As others have mentioned, this doesn't take into account any newer upgrades or pieces installed post-purchase, but would give you a good place to start with.
If you have not reinstalled windows, you can always run:
wmic os get installdate
On Apple machines it seems to be a matter of analyzing the Serial Number, as seen in Ask Different's Where to find factory list in Apple serial number:
Original article that contains the image at the Internet Archive: How do I decipher the entire Apple serial number on my Mac? What does it mean?
Download & install a program called Soluto. I have used it for the past couple of years since it was created, initially to select which programs start up at boot, thereby reducing boot time. Since being first introduced it keeps adding new features.
One of the features is that it tells me how old my computer is.