Following YouTube's acquisition in 2006, Google given two choices to new YouTube signups: "original YouTube account system (built in 2005) or Google Account system". In May 2009, Google started requiring all new YouTube usernames to be powered by Google Accounts. And now, "the company is gradually asking all remaining pre-May 2009 users still using "older YouTube" account to connect Google Account to their YouTube username. For now, it's an optional step, but eventually it'll be required in order to continue using your YouTube username," stated Google.
If you've an older YouTube account you can visit this page and choose to link to an existing Google Account or new.