I've cared for 90 year old men that look younger than Caesar does in game. I understand that 55 would be considered old by Legion standards, but certainly this isn't the case for the rest of the Wasteland right?

what you're saying makes sense, but in the wasteland there would be a lot less medical care available, vaccines and such, that would make life a little shorter i suppose, but in all honesty 55 doesnt seem "old" at all.