It is less about age than weight and reach. A ratproof feeder requires a large difference between the reach and weight of a rat and that of a chicken. One of the many Chinese made treadle feeders like the Grandpa feeder relies only on the weight difference, well, other than the ability of something to push the lid up and crawl inside which is why the reviews on Amazon state that the Grandpa feeder isn't actually rodent proof. The lid has to be super light and balanced to avoid killing too many chickens so sometimes the rodents are smart enough to get in, coupled with the need to leave the feeder propped open for weeks during training....
So, to try to answer that excellent question I have to ask you a question; how ratproof do you want the feeder to be? With the latest versions of our medium feeders the spring tension on the door is adjustable so you can balance out the treadle/spring tension on the door to where a one pound poult could use the feeder but it wouldn't be smart. Better to wait until the poults are three pounds in my opinion so they don't get stuck inside the feeder and die. Like I said, never use treadle feeders when chicks or small poults are in the same coop.
Then there is the reach. The ratproof feeder is designed for full size hens like a Rhode Island Red. It can be used with smaller/less long birds by adding a duck step. But you are eliminating the reach portion of the two reasons the feeder is rat proof. So, set the duck step as far back as it can be and still allow the small chicken to reach the feed. Move it back more if possible once the training period is over.
If you are not yet overrun with rats or wild birds like pigeons, you can likely get away with using a duck step and a light setting on the spring pressure. Just remember with the soft close the spring pressure has to be strong enough to compress the soft close cylinder and pull the treadle up and the door closed so you might have to start off with the soft close cylinder screwed up out of the way if all you have is very light hens. If you have a mixed flocks, the smaller birds will quickly learn to eat when they can.
Sorry I can't give a simple answer, there isn't one as it is more about weight and size than age. Thanks for the excellent question.