Couldn't modify my previous post. Sorry
From RM2:
ChL&CaL - pg 81 Stat Reduction Due to Old Age gives a method for showing declination in stats for, well... old age, but you can adopt that for a long time off as well.
RMCII - pg 11 Out of Play Characters gives a method for handling PCs "out of play" or in "dead time." The formula given is 5% / month + ME mod + SD mod for a chance the PC leveled up during the downtime and it goes into more detail based on category of skill and stats.
RMCIV - pp 30-31 Skills for Very Old Characters The section starts off referring to a character who has been alive for 1000 years and decides to start adventuring, but the info that follows shows stats and info for a much "younger" PC ranging from 21-50 years, 51-500 years, etc.