i imagine that they have SOME sort of maximum life expectancy. Transilvito's Capitol is very well defended, and Don King doesn't seem to be the kind of king who'd go "lead the troops", yet having a high quality Heir Designate was important to him. also, no units are popped as old people, and yet Jack Snipe, Don King, King Salive IV and King Slately are all old.
It's possible that some units are popped with an"aged" appearance. Also, I don't think Jack Snipe looks particularly old except in his bearing (which could be the result of stress -- he was linked for a long time and deranged by the link-snap for most of the time we've seen him, after all), and all we have for King Slately is a third-party description (which also seems to refer to his personality rather than his physical appearance).