In my experience, after a few generations and everything is going good, if you have some peeps in between 60-65, make sure they have plenty of groceries, praise a few non-work/recreational activities and then let them be for awhile.

This is really the only way I've ever gotten one to be over 65. I usually will come back to both the goal having been reached and to find the peep passed on.

I don't know why, but if I check on them regularly or keep the food minimal or have them working even after they've slowed down, it's just a recipe for disaster. I just lost a 59 year old because of this. For some reason, letting them retire comfortably and then letting them live without my constant supervision seems to do the trick.