All the Sleep Token fans I know are pretty old, but that might just be because all the younger music fans I know are more in the rave scene.
Since OP made a comparison to Ghost, I did notice at the last show of theirs I attended, the crowd was very diverse. I saw kids and seniors alike.
Maybe part of the popularity of these bands is that they operate outside the traditional boundaries, so they're more likely to have cross-generational appeal. That helps boost popularity overall. We're in an era now where music is less defined by generations than in the past. As a kid I was mostly exposed to what my peers were listening to, but now I can access everything. It's getting much harder to pigeon-hole fandoms.