Senescence occurs to every character at roughly an average speed of 4 Seasons (84 days, as each season has 21 days) per age. Wounds and especially serious wounds will speed up the aging process dramatically. (For information concerning the old aging system:)
Age does not currently have many effects on characters, but there are some to encourage players to retire old characters and start again with new ones. Here's a rough overview of the age effects:
Skills that are not kept sharp and trained will gradually deteriorate for all characters, that is a normal process. However, older characters lose skill points faster. Again, the effect is not major, not an order of magnitude or anything like that, but it is enough to make keeping an old character sharp a challenge.
Older characters have a greater chance of staying in a seriously wounded state for a longer period of time.
Age is not simply a measure of time. Age reflects how old your body feels. Battle and being wounded cause the body to age more quickly than one would simply lounging around in front of a computer.