Are they playing against other humans?
I got AOE2 HD a couple months ago and it's been my go-to game this summer. But I haven't thought of looking on twitch or for multiplayer online.
There is a 60k tournament underway. Tiny compared to other games, but respectable considering the age of this game.
The game's audience is mainly international. Large player bases in china, Vietnam, EU, and south america. The community can mainly be found at aoezone.net. There is a smaller community also on reddit.com/r/aoe2