Think reach and weight, not age. Depending on the feeder you bought the weight to trip the treadle might be 1.5 to 4 pounds, the pullets will need to weigh more than that or you will need to adjust the balance on the feeder.
Some treadle feeders have those wide, close in useless steps, useless for being rodent proof. Others will have a treadle step 8 to 9 inches away so the pullet will need to have the body length to stand on the treadle and reach the feed.
You can make your own duck step out of something lightweight. Grab a political sign, the corrugated plastic kind, some 3/16" or 3/8" thick plywood, use screws to fasten it on the distant treadle. Cut a bit off every two weeks till you get back to the original narrow and distant treadle.
Make sure the feeder is securely fastened to a wall or post so it is rock solid, put three patio blocks or pavers under the feeder so there is a deep platform, the hen can step up on the first paver, then easily step onto the treadle. They usually stand inside the treadle step with the back foot on the treadle.
Remove ALL other feed. no free range, no treats unless you toss them in the lower feed tray while they are watching. Train in short bursts, one or two minutes. No luck, stop. Go away for an hour, let the hens get hungrier and tray again later. Never give in, never hand feed, if you do you are teaching them that being stubborn pays off. Let them go to roost hungry if the weather isn't brutally cold. The next morning they will be motivated.
Good luck, post pictures if you need further assistance.
