Mac OS X Yosemite and El Capitan

1. Open the console window in the Utilities folder, as outlined above.
2. Navigate to /var/log, then click on install.log
3. Scroll all the way to the top of the list, until you see a series of text that says "OSInstaller" 
   
4. The date next to the text is the install date.

MacOS Catalina

1. Open the console application found in Applications > Utilities
2. In the sidebar select "Log Reports"
3. In the top half of the window select "install.log"

4. The lower half of the log window scroll up to the earliest possible date to see the install date.
   
   

   Note

   The earliest date will either be when the computer was first set after being purchased or from the last clean install. If a computer is upgraded there will be an install log on the date of the install recording the upgrade.