A good idea would be if units slowly would lose valour points (one per 5 years?) to represent the retiring of old veterans and the influx of young recruits to replace them.

That also prevents a TW problem that really ticks me off- it's easy to dominate any battlefield wiht a few 6+ valour units. Particulary with missile and phalanx units, who are easy to use without risk of them getting mauled (and thus, high experience men getting kiled)
In EB, I just disband slingers when they reach valour 5, because units that good lead to incredibly unrealistic (and unfun) battle tactics.