Like the definition of consonance or dissonance, it is all subjective. One young student pronounced a half-step sounding together as "thrilling."
At his age I would have found it horrible.
What is the difference between his reaction and mine? He was not brought up (pianistically speaking) believing that only major and minor chords sounded "right." He was allowed to hear and respond to the whole sound of the piano.
When did composers begin to take that whole sound into account? I think they did so right from the beginning of the instrument's existence: I hear it in Mozart, in Haydn -- even if only in spots; certainly in Beethoven, most definitely in Chopin, Schumann, Brahms, and Bartok, by which time the sounding not quite "right" had become the source of their compositions.