When I was 14 I ran 6:53 in spring gym class... didn't do sports growing at all. That fall in gym class got down to 6:20. Joined track team and started running. Didn't run the mile at all in that first year, but got down to 4:20 within two years.
15y.o.
60 400m, 2:10 800m
16y.o.
59 400m, 2:06 800m, 4:37 mile
17y.o.
58 400m, 2:02 800m, 4:20 mile
58 is still my 400 PR. I ran 14:30s for 5k last year at 19y.o.