I think the limit of 50 Elders is a programming problem. To store the fact that potentially hundreds of villagers have become Elders would maybe take up too much memory and quite likely slow the game down, or even crash it. There's a similar seemingly arbitrary limit on displaying villager's parents in their details screen; when they get to 14 the names of the parents no longer become available, and when they reach 18 the parents' images disappear, too. The other limit, of course, is on the maximum population, and I suspect similar issues are involved there.