I propose allowing thousand year ginseng to be castable upon any commanders. The caster can select a commander to reduce the age.
Thematically, ginsengs are rare objects usable by anyone. The emperors won't be keeping the imperial alchemists on the payroll if the ginsengs do not affect them.
That said, I think the gisengs and internal alchemy should have a much more drastic effecitve to be worthwhile. Say reduce age by 20 and 40 respectively.