You can't really tell by how old they are...its more contingent on their size. When the pinkies start getting too small...and you should know when their girth gets wider than 1.5 the size of the pinky then its getting to be time to move up size of the prey item. Usually before you move up in size of prey item, you can feed them 2 smaller items. i.e. when moving from pinkies to fuzzies, you might take a couple weeks of feeding 2 pinkies and then switch to a single fuzzy. Makes it easier on the transition.
Hope that helps a little.