Just curious, do half-fiends age at a regular rate, or do they grow old and die around 100 like normal people?

I would have to say it depends on if you want your Half-fiend more fiendish or... humanish (or whatever other half there is). I think you'd be able to get away with aging however you want since its only a roleplay and age doesn't matter overly much. If you need your character to be 150years old for the experiences he's had ect. It shouldn't be a problem.