The title says it: the Timeframe of the game is set to 4 turns = 1 year, but Characters age at a rate of 2 turns=1 year.
Easiest way to fix that seems (for me) to include the following text into the Campaign_script.txt:
That it works you also have to change the startdate in descr_strat from "2954 summer" to "2954 winter".
Both files can be found in the mods\Third_Age\data\world\maps\campaign\imperial_campaign folder.
You have to start a new campaign that this script works.
All credits for this script should go to the user GreenEyedDevil, at least he is the one I've stolen it from 
Edit: to make it easier: just download this two files and overwrite the existing ones.
(And of course, make backups from the originals, just in case you don't like it)
EDIT²: fixed some compatiblity-issue with the garrison script, should work correctly now.
Please report if you find any further bugs.
Edit³: Updated the attached files to the current version (1.1) of Third age total war.