1
1. Those with live arms, that you groom to effectiveness.
2. Those blessed with a true plus-plus, wipeout pitch.
.................
Fields according to all reports has a Tom Gordon overhand curve. That made him a great bet to wipe out pro's even at 91 mph; Gordon was at one time MLB's best closer* throwing only 90. Daimajin was superb even at 90-92, because of his 'thang'.
But Fields was then reported to throw 94-97. If he has both of those pitches, you're talking Bryan Harvey or something.
Getting a kid to throw two pitches for strikes isn't rocket science. There's no reason to set a timetable that applies to kids with normal arms.