RojoMarz, thanks for the mention. Six months ought to be old enough to use a treadle feeder but it really depends upon what kind of treadle feeder and what kind of hen. Really it is about weight and reach, having the weight needed to trip the treadle and open the door enough to get to the feed and having the body length to actually reach the feed.
Some models like the Chinese made Grandpa feeder and the many Chinese made clones ARE easier for young birds to use because they use wide, close in treadles, ultra light guillotine style lids, and no spring pre loading on the lid. The result is a lid that even a mouse can push up to get into the feeder because it can stand on the treadle and reach the lid or with the help of a few of its buddies just push the lid open enough to slither in. And for a lot of flock owners that start off without a rodent problem they might get lucky and never have a rodent colony build up. The downsides is that wild birds can also gang up on that close in treadle and steal feed or eventually having a rat colony.
But, if you want a rat proof feeder or a squirrel proof feeder you gotta get that treadle way out away from the feed, keep it narrow and depend upon the grasping of the chicken's feet instead of a wide comfortable treadle step, and you HAVE to add springs to pre load the inward swinging door to prevent critters from just pushing the door open.
We found that over the years a single spring will defeat most squirrels but if you have large amounts of squirrels in the area with overlapping territories (and that is always due to a huge supply of non natural feed being available, they fight over territory otherwise), then you need at least two springs. We switched to dual springs as standard back in January after some beta testing and set the springs directly on the door instead of on the door crank, increasing the leverage and putting the wire treadle link only under tension.
So think reach and weight, not age of the pullets. The good news is that while having chicks and young pullets is unavoidable in a flock, generally if you practice the first step of rat control which is sanitation/feed security, you will NOT have a rodent problem in those few weeks that chicks and young pullets are needing unsecure feeders. And if you don't batch replace a flock but trickle in a few new birds to keep the flock's average age constant the smaller birds will quickly learn to eat when they can. Albeit with a bit of risk as birds under a pound are generally not completely safe around any treadle feeder so provide a refuge only they can enter and have their feed there. A soft close door helps with that IF you have already defeated the rodents and don't need maximum door pressure to keep the nasties out of the food.
And on occasion you might have a feed raking hen or hens if you are feeding treats mixed in with the feed or feeding a mixed feed that they like to cherry pick from. You can use a feeder lip extender to solve that feed raking, choke off the feed by blocking part of the throat of the feeder to reduce the amount of feed coming down, or invite a certain bird to Sunday dinner.