My brother is a former D1 coach and currently a pro scout. He didn't let his 3 boys throw a curve until 16. Also didn't let them play football until high school (and dad was an all-state quarterback). Baseball is #1 sport for all 3 boys. #1 son was his high school's quarterback, played basketball, pitched and played outfield. This is his last year of college baseball, he's a pitcher for a Big 10 school. He did have Tommy John surgery 2 years ago.
Son #2 is a senior in high school. Dislocated his non-throwing shoulder one week before his first freshman football game while making a tackle in practice. Got back for last 3 games. After freshman year he decided to not play anymore football as baseball future seemed potentially bright. Did play basketball as he is 6-5 and was co-MVP on team this year. Plays third base and is team's #1 pitcher, but doesn't really like pitching. Will be headed to an SEC school in the fall on a baseball scholarship, expects to play third base.
Son #3 will start high school in the fall.
Bottom line on age for curveballs...everyone has a different opinion. How's that for being decisive. LOL