Actually, Spotify is very common and used by a lot of casual music fans that don't want to pay for music. We are in a far more tech savvy world and if you look at the intro pages to Spotify and the commercials, they are pandering to the casual Top 40/ClearChannel station listening-to music fans. This is far from some underground music geek only environment, so bsman is spot on:
The sample size of music streaming services (not the number of users/members) is too small for this to have any weight. That said, and reiterating what I mentioned above, Spotify has a continually growing user base, so it may become a better/the best evaluation tool for these types of studies in the future, but I'm not sure it's there yet.
Despite the above, this info doesn't surprise me and based on a bunch of folks I know, 33 years-old seems high. I know folks that basically stopped listening to anything new in/after high school, college, 2000, etc. Conversely, we all have family, friends, acquaintances, etc. that are always listening to "new" "music" because they only listen to Top 40 stations and are fine with being spoon fed and basically being told what to like.
In the end it doesn't matter to me at all, because I'm always keeping an ear out for new and new to me music, while still loving my archives. This week I was exposed to Phox and the Family Crest after following a link from here to a Tiny Desk Concert performance. It was either the Punch Brothers or the initial concert led to me seeing that they had a new Punch Brothers TDC. So while listening to that I was checking to see what else was there (seeing the write-ups and listening to the first few minutes of a performance).
I hope I never hit that wall of not liking new and different stuff.