This achievement description is a bit lacking. Apparently having a hero of exactly 60 years of age still works. Also, everyone has to survive the battle. That's important.
If you play as normal and recruit a lot of people as well as make sure your keeps are well maintained, you should eventually reach a point where enough old farts are still alive when a battle appears. If you have a choice, avoid any battles with Wrinklers, as their aging attacks have a very high chance of straight up killing your people.
As for the fight itself, your guys will probably all suck, unless you get lucky and one of your high level heroes happens to be of appropriate age at the time. Play carefully, save a lot and use health items, and you should be alright.