It could be handled as just a bonus for old characters, i.e. when you get old enough you start learning skills a bit easier. Along the same lines there are SO MANY different skills/spells/supps for most classes that using and improving all of them during ranking is not feasible (besides, many abilities don't lend themselves well vs. mobs, but are key in PC combat), so I think heroes should be exempt from the level restrictions and be able to improve skills by fighting even low level mobs (though at a somewhat slower rate than against tougher mobs... basically how it was before this change).

By the way, if it's not in already, you should outline the system in a helpfile somewhere (help practice maybe?).