My turn :-)
I'm 40, started using lilypond a couple of years ago, I think in 2012, not sure actually.
I play guitar as first instrument and teach it in non-traditional schools, mainly because I don't have the qualifications on paper and now it's too late/expensive/time_consuming to get one.
I use lilyond not only for the quality of the output but also because it is so feature rich. I mean, a lot is very hard to achieve or not achievable with other open source tools and the open source element is important for me.
The fact that there seem to be very few 20-ish people (or none, I didn't go through the entire list here) using lilypond does not surprise me.
And this is in my opinion not because of the way lilypond works (text-based).
Teaching guitar to younger people and older people shows a clear difference between their interests which could be extrapolated to much more (such as choosing lilypond).
Younger people want to see results, without knowing how or what's behind it. I give them a song to study, tabs, sheet music, whatever, and they're happy with it.
The attitude from older people is more like "what is happening there... " or, "why do you this or that." (both in technique as in background theory of the song).
And I don't think it's because of the "age". I think it is because of what people are used to. Now there's an app for everything. With a tap or a click on a button people get "something" without understanding the background. There's a tool for everything and more important, there's no need to understand it. And if they want to understand it they find a link to a wikipedia page :-)
People from an earlier period have known the time when dragging a picture from explorer into word was not possible yet. They (I, or we) had to think about clicking on the right buttons, and to memorise it I had to understand the purpose, or the reason behind the button to import an image into word.
Now I've seen countless times people dragging a picture into word and press the printer button without even thinking that word is not ment for printing photo's, nor thinking about the quality setting in the printer dialogue. But they are happy with it, because they have their photo on paper and that's all they want, not a professional photo print.
Similarly: lilypond. You have to think about what you do with it, you have to know the reason and the idea's behind the program itself to be able to use it properly. Most (younger) people these days don't need professional quality sheet music in the first place, they just want some result, no matter how bad the image quality is, as long as they can read the music, it's fine.
So why bother learning it?
The older way of thinking is not a bad thing of course, I prefer it too :-) . In the case of lilypond, I learned a lot about music terminology, chord construction, instrument specific techniques and signs simply because I needed to find it in the documentation.
For example, for some signs I knew what they looked like, and what they meant, but not the right word for it (probably a language issue too, most of the web is English, and I'm Dutch, so confusion can kick in with a heavy beat). What I mean is that software with a big learning curve makes you learn about more then the software alone. It makes people think in a certain way and creates bridges between different subjects (e.g. the entire list of emails about the e:5 chord, what it means, should mean, might mean, now outputs, shout output, etc.. )
And to get on-topic again, this also makes a program language a beauty :-p.
grtz,
Bart