Proposed Canon
The oldest known unit is King Slately who has ruled for roughly 3000 turns and was likely a heir for an unknown period of time before that. This means he has ruled for over 10 years but appears to be at least in his sixties, however it is noted that units pop fully formed as adults in Erfworld so there is no telling how old Slately appeared when he popped or if natural Signamancy has played a part on him for feeling like an old man just as it's left him short and rotund for avoiding battle.
The average natural life span of erfworlders is unknown. However, there are two major theories prevalent among fans. One is that erfworlders don't age and will only croak or become disbanded by outside influences. The other is that erfworlders slowly get older with each passing turn. And if they survive for enough turns, they will eventually croak naturally.