It was hard a few decades ago to keep track of a player's records, there was no internet so the games were only kept on real kifus, and that always gave me the unconscious feeling that pros were born pros. These days, it's become easier to find post-pro records, even though it may still took pretty long...
So, to not forget they were not all born pro, a few samples.
First game : Cho Chikun(W) 3k vs Kobayashi Koichi(B) 1k
It was played in 1965, when Cho was 9, (he was an insei since he was 6) two year before he became pro.
Kobayashi Koichi was 13.Reached 1p in 1967, the same year as Cho.
Second Game, played 23rd October 1965, between Cho Hun Hyun 1k(w) and Cho Chikun (b) 3k. They were both inseis and both were promoted as 1p in 1967 as well.