I just wanted to post a succession to make stats gain dependent of the fighters "level" but as i see i have been preceded.
Anyway what i tought is that you calculate stat gain multiplier (like heat) for the attacker (only attacker) based on the difference of age,amount of crits,number of fights fought,stats, (age being the least relevant). That multiplication can also go to negative at some point. That would give the new chance to catch up and protect the from win farming.
In any case is see this as being quite logical. The young have more to gain experience wise from fighting old players than vice versa.