It is roughly at 300 hours that you become "old". Even after that you probably still have at least 100 hours left in you, and 200 or more is not unlikely.

Being old isn't much of a problem for a mage. It is dex and strength that take a big hit, so it is the fighting classes that suffer, particularly those that don't use stealth.

Some of the (age deaths) you see on the battlefield are from premature aging. For example, there is a sword that makes you age faster, and, in my case, my character's body gave out during a magical ritual and was listed as age dead at 320 hours or so. However, I could have lived quite a bit longer had I not conducted the ritual.