Will this count for anything in my college app (not planning to go to a music school)? Or does anything before high school not really matter?
This will really just depend on the application and how you want to handle it, and what else you have to put on the application. For example, if the application says to list only achievements that occurred starting the summer before 9th grade (like a lot of applications do), then you would want to follow the instructions and not mention the performance directly in the application form. However, there would be nothing stopping you from mentioning it in one of your essays, or even writing an entire essay about the event and what it meant to you, how you prepared for it and what you learned from it. For essays, there is usually no “time limit” on what period of time in your life you can write about or not. Another way you can slip something like that into the form itself is if the application asks for hobbies, and you are still playing piano as a hobby or taking lessons, you can mention the performance indirectly in your description of the hobby (“played piano 10 years and performed solo at Carnegie Hall”).
Will it impress college admissions, even if it is not related to your planned major? It can if it is handled correctly, like in the descriptions above, where it can show long-term commitment to an activity and very high achievement. It can hurt if it is handled incorrectly, like putting that as an “honor” in a section that asks for what awards and honors you got in high school, because then you wouldn’t be following directions. Make sense? Just wait and see if seems appropriate in a particular college application, based on what is asked for and what types of essays they want, and what other achievements you have to put on there by then.
And keep in mind that playing at Carnegie Hall in and of itself isn’t necessarily impressive. Anyone can rent Carnegie Hall. You need to put it in context - what group were you part of, and is that itself prestigious; is being given a solo by that group a big deal, etc.
If you have continued the talent/instrument that you performed at Carnegie Hall then you should have other more recent achievements that would be better to highlight in a college application.
The American Protege International Competition, for example, operates to produce literally dozens of “winners” (from EACH of four or five series of so-called competitions a year), then gives all the winners an opportunity to pay hundreds of dollars to “perform at Carnegie Hall” that they rented. There’s gotta be hundreds of students who put “solo performance at Carnegie Hall” in their college applications enough by now for the adcoms to see through this. I’m sure these adcoms know the difference between this kind of “Cargegie Hall solo performance” from those given by internationally well-known virtuosi. I personally know a few in my state who have “won” the online American Protege International Competitions and performed at Carnegie Hall while unable to win anything of significance in the state competition. I also know of a good friend of ours that won 1st place at this competition, found out what it’s really worth, decided not to bother going to the Carnegie Hall and decided against even mentioning it in the college application in fear of besmirching her real reputation.
@TiggerDad is spot-on. Of course, having your own solo performance for the entire night in the Stern auditorium of CH is entirely different.
Be a good and passionate musician. Submitting a well-prepared audition tape is the key if you want music to be the admission hook for those universities that care more than GPA and SAT. There will be someone, most likely a music faculty or conductor, evaluate your audition videos at those schools.
My S was invited to play at the Weill Hall of CH, and music was his hook. But the schools admitted him could care less about the Weill Hall performance. It was his audition tape and some other stuffs put him through based on the feedbacks from a few admission officers.