Is there a way to get the date of the first boot of an old mac?
The closest answer to my problem could be:

ls -lt /private/var/db/ | tail -3

which shows for example:

drwx------ 8 root wheel 272 Apr 18 2009 krb5kdc
drwx------ 3 root wheel 102 Apr 18 2009 dhcpclient
-r-------- 1 root wheel 48 Jan 14 2009 SystemKey


Ignoring the date of the "SystemKey"-file.