Age of my computer

Did you read the following from that link?

Your Mac provides several tools to help you identify it. The simplest is About This Mac, available by choosing About This Mac from the Apple () menu in the upper-left corner of your screen. The other is the System Information app. Learn how to use these tools to identify your Mac.

If you don’t have your Mac or it doesn’t start up, use one of these solutions instead:

• • Find the serial number printed on the underside of your Mac, near the regulatory markings. It’s also on the original packaging, next to a barcode label. You can then enter that serial number on the Check Coverage page to find your model.
  • The original packaging might also show an Apple part number, such asMQD32xx/A (“xx” is a variable that differs by country or region). You can match the Apple part number to one in the list below to find your model.

Enter your Mac's serial number here:

http://www.appleserialnumberinfo.com/Desktop/index.php

One of the line items it will return is the date your Mac was manufactured. Not just Apple's model date.

The following may help: Identify your MacBook Air model - Apple Support

Posting your serial number online is a very bad idea...

That is a mid 2013 model.