I've had an account here for 4 years, but only recently started using the site. And now I'm receiving Yearling badge one by one each day.
Why don't you give all of them at once?
The rules for the badge are, you have to have earned 200 points for each year you've been here to qualify for "that year's" badge. Until two days ago, you didn't qualify for any of them. Then...
When you cross 800, you can expect to get your fourth Yearling.