There are a few but I'm not sure there's enough to make an empire out of.
21 and under will give you a bigger pool of possible members, but don't underestimate the potential of a small but sturdy crew.
The Gerontocrats have only had 22 members ever, and only a couple of us have been posting games so far this year, but we are still #7 in the official Metaverse and #8 in the
AltMeta. Even the top empires owe much of their status to the strength of their leading players, not the sheer number of members.
Besides, there's no rule about what an empire needs to be other than whatever the founder and members agree to. Someone could found The Ultimate Losers and throw out anyone who ever posted a win.