The following user on SO has a reputation of 91 (from one question) but still has a yearling badge. This seems to go against the description of the badge which says you have to have 200 rep per year.
The other answers and comments cover this in bits and pieces, so here's a comprehensive answer. Maybe too comprehensive... but I'm hoping to use this as a template of sorts for future reports of this issue.
The behavior of the Yearling badge was changed in December 2009. (It's my fault.)
The user in question, @screepnick, registered on Sep. 18, 2008, the same day he posted that question, and he hasn't really had any activity outside of that question. Per @mmyers, he has no deleted posts. I can't verify this, but it looks like all of the activity on the question took place well before his first anniversary. Currently, the question has 18 upvotes and no downvotes.
With those 18 upvotes, @screepnick would have had 181 rep on his anniversary, well over the 100 he needed. This would be true even if a handful of changes in votes came later. And there was roughly a three-month window between his anniversary on the site and the time the badge's rep requirement was changed, more than long enough to account for any delay in the badge awarding cronjob.
Badges, once awarded, are never taken away solely because of changes in the requirements or conditions. (EDIT: this rule, too, has an exception; Beta badges were unawarded at one point when the requirements changed.)
In summary: it's not a bug, it's a valid badge! Hurrah!
One thing to note is that before the rep recalc the user would have had 181 rep plus any rep from deleted questions.
So at one point he has been eligible, and as you know, once you gain a badge you don't lose it again, unless it's deleted by jeff.
I would expect that the criteria changed over time. This user has been on SO for almost 2 years, so the badge was awarded some 11 months ago. I recall that at that time only activity was requested, not a certain amount of reputation. Any long standing SO members to confirm this?
Recalcs and rep changes (5 instead of 10 points for questions) might have caused this too.
Anyhow, wait another 16-17 days and I assure you that this person will not have a second yearling badge, unless you just pointed enough people interested in MSIL and Java byte code to his question, or he gets busy.