I suppose when I was 17 and in a band I would have defined it as having gold/platinum records, living in a mansion, riding in limousines and the like but isn’t that just being successful at playing the capitalist game and shifting units? This is a business like view of art. When you filter art threw this business model (to which everything is subjected) you get shallow crap such as Brittney Spears . I think if anything, that stuff obscures what being a musician is about (to me at least, now as a slightly more mature 24 year old).
Asking “what is musical success?” is like asking “what is the meaning of life?” completely subjective, shifting and maybe even abstract. Perhaps I can obtain musical success right now if I put on a Beethoven or Hendrix CD and am genuinely moved by it? Or if I pick up a guitar and express myself?