We have that, it's called "age". The age attribute in BM does not represent calendar age, it represents physical age and you DO "age" when you get wounded.
Still, I do like the idea, and we could convert between the two.
I like the idea of mortality at 'old age' (from 60 to 90-100?) ! In RL, I often had meet people relatively young but totally 'worn out' (and the opposed). The only thing... maybe it would be good to show both ages, Chronological and 'vital'?