I don’t know if there is a definitive ‘zodiac’ for the hyborian era - there wasn’t one written by R.E Howard, at least as far as i know. However, given that Howard’s world is so compelling because it so firmly based on the real world I would use the Chinese Zodiac and replace the Tiger with Lion, and the Snake with Cobra.
That sounds great! I would love to read your synopsis if you ever finish it! I have lots of Howard literature in my study, but I don’t have the time to help out with the ageing question just now. In about a month I have some holidays scheduled…I could do some research for you.