In 1957 when I was 17 and still at school I was buying 78s of Bill Haley and his Comets. An older boy, who must have been all of 18(!) took me aside and said, "You shouldn't be wasting your time with this stuff - you should be listening to jazz" and pressed on me an EP of Sidney Bechet with Claude Luter's band, which I liked. Being a rather academic boy, I looked for a book and unfortunately found Jazz by Rex Harris, who was a hard-line traditionalist who basically considered anything that didn't have a banjo in it to be too modern to be considered jazz. So I bought records by Humphrey Lyttelton, but also LPs of King Oliver, Morton and Armstrong. So I was cut off from most of jazz, not by date, but stylistically. And it wasn't just as a result of Harris' book. I remember borrowing this Hampton EP and thinking, "I just don't understand this music"(!)
But by the following year I'd heard Bird, Miles and Monk and had sufficient musical maturity to dig what they were doing and didn't look back. So, once again, it wasn't dates, but styles that mattered for me. Of course, all this took place against a background of the war between "traddies and modernists" that raged in British jazz circles in those days.