Random question: Do you think early-to-mid-20s is a realistic age for someone to be the captain of a spaceship? Bear in mind this isn't an official ship with academy training and the like required, it's an unregistered ship with a crew of what are essentially freelance criminals.
Because I instinctively write MCs young but I'm worried that if I go too young with this guy then it won't be believable.
The way you describe the ship, the age might work. Could be justified as a mutinous crew, training vessel, privateer, or good ol' fashioned band o' pie-rates.
That said, there are certainly cases where young whippersnappers can be captains. If the actual captain was killed/removed somehow, the first officer/mate might be frocked to the rank of captain. War, especially a bloody one, often sees rapid promotions and a lot of young, ranking officers/enlisted in charge of massive units or large vessels.
Size of ship can play into this as well. An aircraft carrier would have a more seasoned naval officer at the helm, but smaller Coast Guard ships can have "captains" with the rank of Lieutenant and Lt. Commanders. A junior LT tends to be around the age of 24-26. Additionally, Yard Patrol craft (training boats called "YPs") at the US Naval Academy are also "captained" by junior officers.
Finally, PT boats (made famous in WWII), were captained by junior officers. They were small, fast attack ships armed with torpedoes, guns, etc. and utilized a lot for ambushes. During WWII, President John F. Kennedy captained one as a Lieutenant Junior Grade (LtJG) or O-2 (one rank above an Ensign).
