I don't know about the age thing on Twitch. I think most of the streamers I watch the most of are my age or older, or at least 30+. I don't have time for 20 something shenanigans as I might have done 10-15 years ago. Here are some of my favourite:
Squirrel - A Brit about 60 or so, mostly streams sims, such as Euro Trucker. Usually has 2-3k viewers.
Shenryyr - Around 40, US variety streamer. Mostly streams Paradox games and industry/rail road games. 500-1k viewers.
Malic_VR - 40 something US dude, mostly streams VR Elite: Dangerous as well as lots of DIY stuff. Low 100s viewers.
Father_Bill - 50 something US Catholic priest. Streams mostly Elite: Dangerous as well as some Mech games. Low 100 viewers.
RedWeird_ - 40 something UK variety streamer. Low 100 viewers.
Muckshifter - 70+ UK streamer. Mostly plays LotRO, as well as some other games. Sub 100 viewers.
There are more, just couldn't be arsed to list them all. Now, they all have different personalities, but they're all the same in that they are mature, and cater to a mature audience. They're laid back, discuss things level headedly, there there to play a game and interact with people, not to "be a star and make millions" or whatever. I think Shenryyr is the only one who earns his living off Twitch, but he rents a room and seems happy to be ForeverAlone. The rest either do it as a hobby, or are semi- or permanently retired.
Sure, there are a few younger ones I also watch, but they usually also cater to older crowds.