Feller's rookie year was 1936 and yes he was only 17 years old then.
1938 was his first full season as a SP (he logged 62 innings in 1936 and 148 innings in 1937) and he pitched 277 innings with 240 Ks and 208 BBs as a 19 year old.
Feller would have easily totalled over 300 Wins (he finished with 266) if not for WWII. He was a 20+ game winner in 1939, 40 and 41, missed all of 1942, 43 and 44 and barely pitched in 1945. He then picked right back up with 20+ win seasons in 1946 (with a then All-Time best 348 Ks) and 1947 but fell just short in 1948 with 19 wins.
He would have easily had an additional 60 Wins and 600 Ks if not for WWII.
He wasn't called Rapid Robert for nothing: