Children have to put in the hours, maybe the same or slightly less than an adult does, to learn to read music. A 9 year old will need to practice reading and playing daily for at least 3 years before they can kinda sorta (sight-)read through beginner material with some facility. Most (middle-class with supportive parents) children who are learning to play an instrument benefit from having the time, space, and energy to do this as a part of their ordinary schooling routine. This is of course in contrast to many adults who have to attend to work, family, and other life things.

I started learning to play music as an adult with zero training as a child, and in my observation, adults (such as myself) don't actually have a problem learning sheet music, so long as they're comfortable practicing reading for several years daily, just like a child would. I'm several years in with daily practice, and I can work my way through sight-reading early-intermediate classical repertoire, albeit slowly.

Adults, at least those with the privilege of learning music, are usually already quite literate and perhaps even quite formally educated. Moreover, said adults also have a strong conception of music—their ears are good and attuned to their preferred styles of music. To many such adults then, it feels agonizing to start learning to read, and dedicate oneself to the pursuit at a child-level for many years. No doubt plasticity is a factor, but I genuinely think it's grossly overstated.

> I’ve tried several times to learn sheet music in my 20’s and it is brutally difficult.

What do you mean by "learn it"? Just being able to decode what notes are supposed to be played and how long they should last from sheet music, given no time limit? Or doing it in near real-time?

Is there any point to learning to do it not in real time?

Well, of course. I can only do it pretty slowly, and I can still learn songs, intros, licks etc. by finding sheet music online and spending time with it. Isn't that pretty obvious?

Isn’t that essentially the same as memorizing the notes for the whole song?

I dunno, I guess it’s useful, but if I were trying to learn to read sheet music it would be so that I could pick up and play.

I'm pretty fluent in reading sheet music. Still, for most songs the process is figuring out what notes to play in sub realtime, then slowly speeding up where both reading and fingering are limiting my speed. Then, as I "know" the whole song, looking at the sheet music helps my fingers remember which notes come next—together with muscle memory. And here sheet music notation shows its strength. At this point I don't see individual notes, but groups of notes, both in time and in harmony, and because I see them as groups at once, my sight reading can keep up with the song. But that takes practice for each song.

The latter, obviously.

Pianists call real-time reading "sight-reading", and it's not what pianists usually focus on. Even the most skilled pianist is probably not going to sight-read a Bach fugue (or be satisfied with the result).

The more common way to read music (at least in terms of the large and marvelous classical piano literature), is as part of a careful process of studying. You read to learn and understand the music, but the reading doesn't need to be real-time (it normally isn't, when learning a piece).

Real-time sight-reading is its own special skill, which you can practice for its own sake. But it's nowhere near as important as the non-real-time learning process mentioned above. I got pretty good at sight-reading when I worked as an accompanist, but I still hate it: It's not how I like to learn a piece. (And the music I find most interesting [like Bach!] can't even really be learned that way).

It feels like most of that frustration is (or should be) directed at other commentors. I tried to frame my questions in a way that someone who does not read sheet music and is also unfamiliar with tabs might frame them. I saw the developer had been replying to comments or I wouldn't have bothered. I was hoping he might elaborate for potential users of his app. I certainly didn't claim it wasn't a good idea, merely that my frame-of-reference gave me no indication of how it worked for piano.

My parents never bought us a piano. My grandmother passed her decidely-not-Steinway down to us, and also paid for lessons with a school music teacher. I joined the band in 5th grade and played a school-issue trumpet until my parents could afford a used one. After my 10th grade the band teacher lost almost all his trombone players to graduation, so he taught me over the summer before I started my junior year and gave me an old trombone with lots of dings, which I used through my freshman year in college, after which I dropped band because it was a 1 hr credit and took about 25–30 hours a week (practice with band, practice on own, perform at games), and I wasn't a music major.