Elly De La Cruz is 20. Noelvi Marte, the centerpiece of the deal that just sent Luis Castillo to the Seattle Mariners, is 20. Edwin Arroyo, the other Top 100 prospect the Reds nabbed in trading Castillo, is 18.
Cam Collier, the Reds first 1st round pick from earlier this month, is 17. Sal Stewart, the Reds second 1st round pick from earlier this month, is 18.
Chase Petty, the prize of the deal that sent Sonny Gray to the Minnesota Twins and kick-started this entire re-re-rebuild, was 18 at the time of the deal, and turned 19 in April.