In CK2 the 17 year old girl have higher martial score than Subtai and Genghis Khan which is beyond ridiculous.How would that be non-sensical? What is nonsensical is sixteen-year-old girl having a higher martial than thirty-year-old who is veteran of thousand battles.
See how it work in HOI4, your commanders get better and better and also get various traits for doing various things which make them even better at doing what they have already been doing.I am of the opinion that battlefield experience should not only raise your dueling ability but your martial too, but IIRC vanilla only supports the former.
You can make it work like statemanship in Imperator: Rome, each character have an attribute level for their potential, so for example Genghis Khan maybe have 30 in all 5 attributes but only have 25% martial experience and thus only have an effective score of 25% * 30 = 7.5 = 8 if round up and maybe 10% diplomatic experience and thus only have an effective diplomacy attribute of 3.