It may take time - it depends on how you play. If you like to be active and force them forward it will go faster. Some villagers are more prone to becoming and elder. They work harder and don't spend too much time in the pond or whatever leisure they have. Keep an eye on those and switch them to a new task whenever they get master in a specific area. The runners are a big advantage: Put them in tasks like farming and building in their youth and then maybe science or parenting. Some villagers just seem to hate a specific skill, for example science. They will then try to do something else as soon as you look elsewhere. It really tests your patience.