I just loaded the SO homepage, and every one of the "Recent Badges" listed down the right-hand side was a "Yearling" badge.
Did I just happen to arrive right after some cron job had run?
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As the others said, Yearling is issued daily, and all those users signed up on 25 Sep (2008 and 2009). Here's a list of the creation dates of all the accounts listed as receiving Yearling on the homepage right now: http://odata.stackexchange.com/stackoverflow/q/11723/