OK, 'Major' Mandella (25, correct me if wrong), makes sense.

'Commander' Skywalker (22) makes sense.

So does 'Generals' Kenobi, Solo and Calrissian in the context of a war/revolutionary situation, as does 'Admiral' Daala (16) as she was sleeping with Tarkin, but giving someone the commissioned rank of 'Captain' to someone who did not even complete basic/officer training does not make any sense .

New Kert Rats (Star Trek) really does not make any sense at all.