AdWords logins began migrating to Google Accounts, mandatory from January 15, 2006

Google started moving AdWords sign-in to the Google Accounts system, so one email and password could be used across Google services. Updating was optional from October 19, 2005 until January 15, 2006, after which advertisers had to convert before accessing their accounts.

Any email address could be used as the Google Account login; a Gmail address was not required, and advertisers could keep separate accounts for personal and business use.