As for who would play the younger Alonzo Harris, assuming the character was the same age as Denzel Washington was when Training Day came out, i.e. 46 years old, that would make him approximately 37 years old in this prequel. Washington’s son, BlacKkKlansman’s John David Washington, is currently 35 years old, so perhaps Warner Bros will seek him out, especially since he already has a relationship with the studio by starring in the Christopher Nolan-directed Tenet alongside Robert Pattinson and Elizabeth Debicki.
Article source: https://www.cinemablend.com/news/2482052/a-training-day-prequel-is-in-the-works