It should be logged in the action log. Go to Extensions, search for the Action Log extension (install it if you don't have it installed) and click the open button. You'll filter for the "Create Mailname" action, which will show al actions for mailbox creations.
Alternatively, you could connect to your server via SSH and simply look for the creation date of the mailbox directory. For example with the stat
command, like stat /var/qmail/mailnames/example.com/nameofmailbox/