Here are the character descriptions and very loose age ranges:
Harold Dawson: Early 20s. Marine who attacks Santiago. Believes in the importance of the code and the corps. Downey’s superior.
Louden Downey: Early 20s. Marine who attacks Santiago. Believes in the importance of the code and the corps.
Matthew Markinson: 40s. Marine with strong feelings about what happened to Santiago. Outranked by Jessep despite being older than him.
William Santiago: Early 20s. Marine murdered at GITMO by Dawson and Downey. Was desperate to leave GITMO before this.
Nathan Jessep: 40s. The highest ranked character at GITMO. A golden boy of whom bing things are expected by the corps. Believes in the importance of the code and the corps.
Jonathan Kendrick: 30s or 40s. Marine at GITMO, and superior of Dawson and Downey. Believes in the importance of code and the corps. Believes God is always watching.
Sam Weinberg: Late 20s or 30s. JAG lawyer, and Kaffee’s friend. One of the smart kids, and often tasked with keeping Kafee in line.
Daniel Kaffee: Late 20s or early 30s. JAG lawyer know for settling cases out of court. Biding his time in the JAG corps until he can become a corporate lawyer. Son of a legendary lawyer and has big shoes to fill.
Joanne Galloway: Late 20s or early 30s. JAG lawyer. The only woman in the show, and in a world of men. Takes her job seriously, but is perceived as a pain in the butt. All heart, but not much experience.
Isaac Whitaker: Late 30s or 40s. JAG attorney, Kaffee and Sam’s superior, who is generally hassled.
Jack Ross: Late 30s or 40s. JAG attorney and prosecutor in this case. A thorn in the side of Kaffee/Sam/Joanne. Very experienced and well regarded.
Jeffery Howard: 20s. Same rank as Dawson. A key witness it what orders were given (or not) regarding Santiago. Model Marine.
Julius Randolph: 40s or 50s. The Judge at the trial. Tough but fair, and understands the world of the Navy.
Walter Stone: Any age. Doctor onside at GITMO who examines Santiago’s body. Struggles with meshing his morality as a doctor with his loyalty to the Marines.
Others: various small roles with some lines, but present a lot on stage. Will get the chance to play multiple parts. May be played by men or perhaps women.