It would be nice if age actually did something though. Having your champion retire from battle and take up training recruits full-time is fine by me, as long as skill rusting is made less retarded overblown so he isn't a novice again in 2 years. Old craftsmen would probably just get slower, less precise or whatever else fits. Genetics should play a role.
You can do that with modding a creature's SKILL_RATES token.
Standard is 100:8:8:16, which means that a creature gains experience at 100% normal rates, starts rusting after 8 turns (including walking from the workshop to the stockpile to pick up another raw material to do the work that will give them more EXP), loses a point of exp for every 8 turns after they start rusting, and if the dwarf is at the threshold to losing a skill rank, will wait another 16 turns before they lose their skill rank.
As has been noted, this is absurdly fast. I went into a discussion about this somewhere around a month ago, but I basically said that it should look something more like this:
[SKILL_RATES:35:120:24:840]
That way, you have to actually not work for an entire day before you actually start rusting. Rust still goes down fairly fast once it starts, with an entire week taking place between demotions. The 35% normal rate of gain was put in to better reflect that it should also take you longer to actually climb up to legendary in the first place, even if it takes longer to slide down from those peaks.
Similar rates exist for Mental and Physical attributes, and should be modified accordingly.