If it has added calcium, then if it were me, I would wait just awhile longer. When they are not laying, and you feed them calcium, it is not good for their kidneys, I have read. And maybe other body areas....you might be able to do a search on here for finding what bad things happen.
Your peeps are just about 4 months old. I think laying begins mostly around 5-6 months, from what I have read on here. (Some of the breeds I have might not begin laying for me until spring.)
When one of them starts to lay, that is when I would switch them to layer feed. Also give oyster shell free choice. Now for some people, when their bag has run out and it's time to buy more, I advise them to go ahead and switch. But that's when their chickens are older than yours.
Your chickens will also benefit from the higher protein content of the starter/grower feeds. (Should be 17%) as opposed to 16% layer.
Mash is an option, but when you buy your next layer bag, consider layer pellets. They are less wasteful because the chickens can recover from the ground the spilled feed.
If you do decide to wait and give them the mash later, be sure to store it in a plastic or metal bin safe from rodents and bugs (and mold). Moldy food can be deadly.
