Marignons mages start with old age. An old age unit every winter has a chance to gain an incurable affliction.
As vfb mentioned,increasing the growth scale can cut down the rate of affliction.
You can also make the units *not* be old, by empowering them in nature. This will increase the age that old age starts at by around 5 years - and in some cases will stop them from being old.
There are also some items which stop aging - boots of youth or elixir of life.
If all of that is too much, I think I removed old age on some of marignons units in my BalanceAge mod, located here:
http://forum.shrapnelgames.com/showthread.php?t=43566