This might work for you, but not 100%. Open Finder and select the username in the left pane (which is also referred to as the home directory) that was the very first account created and then either hit cmd+i or go under the File menu and select Get Info. In the window that pops up open the General section and you'll see a Created field. This should be one of the earliest things created that you can get to easily and should be VERY close to what you want, if not exactly.