Is it possible to read the date when a mac first booted via terminal or system information GUI? Maybe there's a file set at this moment.
Any file can be changed later, but you have to go looking for this one.
AppleCare can also provide you details if you give them your serial number. They have manufacture date, sale date (sometimes inaccurate) and possibly if the user runs the "Mac Buddy" setup assistant and registers the Mac online during the first boot.
The closest answer to my problem could be:
which shows for example:
Ignoring the date of the "SystemKey"-file.