like Morifea said. use the young generals against small rebel armies. make sure you outnumber them by a good margine. i use a stack with 4 young generals to attack the rebels, sometimes supported with a infantry unit or 2.
also good tactic is to use young general unit as cav unit in your big armies, use them for flanking pinned down infantry units. and running down fleeing enemies.
make sure that you transfer good command traits from older generals to the young ones. dunno if its the case but the beter their traits are the faster you get higher stars