Timeless Body is a cool ability, conceptually, but the only time it's really going to be useful is if you start at 17th level or above and play a monk who rapidly gained levels and then waited thirty years to adventure again. I don't think there'd be any balance reason to not have it retroactively remove aging penalties. Also, it would be perfectly fine to let the ability also prevent you from dying due to old age, since that's only ever useful for roleplaying purposes (the image of the thousand-year-old human monk who's still goin' strong is just *so cool*).
(This also applies to the Druid's Timeless Body).