The EM board pass rate goes down with years since graduation.I'm not in EM but I've never really understood why people (who aren't going into sub-specialties, or who have already completed their sub-specialty training) take the boards immediately after graduation (unless there's a significant pay bump as @RPedigo mentioned). You're already overwhelmed with starting a new job and the responsibilities and life changes that come with that. And you can't help but learn more that will help you with a year or 2 of practice under your belt.
I took my subspecialty boards at the start of my 3rd year of practice. I barely had to study for about 70% of the exam as it's what I spend 95% of my day on. The rest wasn't too bad. Passed it with a lot of room to spare.
Might also be due to entrenchment of personal practice patterns which differ from the "book answer".Interesting. I wonder if that's a selection bias since it's more likely (at least in my mind) that people who wait to take it do so because they don't feel prepared and therefore skew the curve.