How do we make sure that our older laying hens don't eat our younger chickens' starter feed, and vice versa, since the two age groups are meant to have different feeds?
Good question! Typically, you don't want your chicks to eat layer feed since it has too much calcium for them, and you don't want your hens to eat medicated chick starter, because the medication can be transmitted to you through their eggs. However, if your birds are kept in the same area, it's virtually impossible to guarantee your birds will eat the right feed (and with typical perversity, they are usually drawn to the feed you don't want them to eat!)
If you have birds of different ages that may have different dietary needs, there is a way to manage, however. You may want to switch your whole flock to a grower developer feed temporarily. Since grower/developer feed is unmedicated—check your brand, of course, to make sure—the eggs your girls lay while eating this feed can be eaten (chick starter often has medication). Grower/developer doesn't have all the calcium in layer feed, so will not cause problems with too much calcium being consumed by growing birds who are not using it to produce egg shells. For your layers, though, you will want to make sure you are also offering oyster shell free choice (separate from your feed) in case they need the supplementation for producing good, hard egg shells. Using grower/developer and offering free choice oyster shell is a good compromise that will meet the needs of chickens who are at different levels of maturity and with different nutritional needs.