I have several bicycle rides from last year and beyond that cover segments but they aren't be included in the leaderboards. Also, when I view the activity it doesn't show the segment in either the favorites or Other Segments. It's set up as the same course from rides that I did last month that are showing up in the leaderboard.
The majority of leaderboard stats were from rides that were done within the last few months but I've also seen a few dated back to 2013. Perhaps it has to do with when they were uploaded to Garmin Connect? All of my activities are publicly viewable as well so that shouldn't be the issue.