It’s not just calcium content, chicks require different protein, amino acid, and iirc much higher phosphorous content in their feed than adult birds. That feed says 16+ weeks because it’s designed to meet the nutritional needs of laying hens, which does include higher calcium.
The Showstopper is simultaneously labeled as a complete feed and as scratch. Scratch is not a complete feed— it’s a medley of other things, typically high in protein, meant to be fed as a treat for enrichment & extra protein. I am not sure why it’s labeled as a complete feed, the ingredients are mostly corn, sunflower, etc. which are very calorie dense.
The other feed, Healthy Hen, is a bit better, but it’s also a “mash” style feed, meaning that the individual ingredients are recognizable and therefore your chickens can pick through the feed, eat whatever they like, and leave behind whatever they don’t. They are less likely to get a complete ration from this style of feed since they’d of course prefer to eat the tasty bits.
Just feeding a crumble/pellet all-flock (20% protein) would be best.
But if you’re just trying to use the scratch/mash style to mix in their regular feed, I’d say there’s nothing wrong with keeping the Showstopper scratch — just no need to mix it in to their normal feed since it should be used as a treat IMO.
Here is a link to a document about poultry nutrition & feed requirements. You can find a table of requirements on page 20 I believe… this may not be the most up to date doc but I am not on my computer and not willing to fight the good fight of searching for these sorts of things on a mobile device lol.