My Black Australorp laid her first egg today; she's 23-1/2 weeks old. My Cuckoo Marans laid her first 4 weeks ago, at age 19 weeks. I'm so sorry for Mahonri; that is a heck of a LONG time to wait.
About changing their diet right now... What's nice is to keep the protein level higher than layer feed for developing pullets who haven't laid yet. Either you can put them on layer feed and supplement with other protein sources in their treats, or you can mix starter with layer until they are all laying. Here's what I did... Just before 19 weeks, I began a gradual switch to layer feed. Actually, I had a partial bag of starter/grower remaining. So, in order to not waste the feed, I mixed the layer feed with starter feed in their feeder, plus I began giving them oyster shell in a free choice dish in their run. That worked great for getting good shells from the CM and keeping protein levels higher for the non-laying pullets.
About when they will lay... Have you noticed reddening in the combs, wattles, ear lobes, and have you seen them squatting when you try to pick them up? They'll usually start squatting at least a couple weeks or more before they lay their first egg. Some pullets will also suddenly get VERY interested in the nest, and spend time sitting there, a few days before they actually lay. If you offer oyster shell, you may notice some of your pullets begin to eat more of the shell. If these things are happening... it is very hard to remain patient because you know that "ANY DAY NOW... " And then, one golden morning when you think they will never, ever lay....Ta Da, there will be a beautiful, warm egg! And you will love the chicken that laid it.
If you haven't checked out the thread called "How old was your chicken when she laid her 1st egg?", you should. There's good info there you may find helpful, with posts about different breeds and age ranges for 1st egg.
https://www.backyardchickens.com/forum/viewtopic.php?id=231744&p=1
