Now, we’re gradually asking all remaining pre-May 2009 users still using the older YouTube account system to connect a 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.
I wish I had known more about the connection between the google username for youtube and AdSense accounts when I joined.
I tried to set up a youtube account but was redirected to google. Wanting to keep my AdSense account separate so that other people could manage the google / youtube account without seeing AS I used a different email.
Now of course, I cannot link AS to the account!
Something to consider if you are about to set up a G account for youtube.