I was also wondering the same thing. I tried my best to keep my villagers as healthy and happy as possible. I've also kept them from doing the more hazardous tasks when they've gotten older.
The best I have done while actually taking care of my villagers is 94 years old. No matter what I did, I couldn't make the old man live any longer. I actually saw the old man die before my very eyes! He was walking from the food bin to go admire the gong, when all of a sudden when he was just past the fire and near the research table, he disappeard... and a skeleton appeared near the fire. I clicked on it and yep... it was the old man, hehe.

The thing was, once my graveyard filled up and I coulnd't keep track of the ages of my dead villagers, I relied on the Stats page to keep me up to date on my oldest villager. My oldest, according to the stats, was 101. So I guess you can't really control much on how old you can get them to live... you can only do so much. I'm sure if you play long enough, you'll experience the same, where you'll get a villager over 100 and you'll never know about it until you check the Stats page, hehe...