Career stats for Sandy Koufax (SK) as of 10/15/57, age 21y 9mo, and Clayton Kershaw (CK) as of 6/26/09, age 21y 3mo.
SK: IP 205.2 W 09 L 10 H 182 ER 91 K 182 BB 108
CK: IP 190.1 W 10 L 10 H 171 ER 85 K 178 BB 94
SK: ERA=3.99, WHIP=1.41, K/9 innings=8.0
CK: ERA=4.01, WHIP=1.42, K/9 innings=8.7
If you don’t think that’s spooky, you’re not paying attention.
(yes, I know SK was pitching at Ebbets Field and doing a lot of relief pitching, and probably didn’t have great defense behind him; but still….and Kershaw is 6 mo younger. Physically they are virtual clones.)
See also this.