That’s a bit of a dilemma isn’t it? Like Den said, they don’t need the extra calcium when they are molting. There are differing opinions on whether or not the extra calcium harms adult chickens. Growing chickens are clearly at risk, but most decent sized flocks will always have a few hens that are not laying. Adults are much better at getting rid of that excess calcium.
Like Cindy, I always have a few laying, even during molt. They don’t all molt at the same time, some are faster than others, and I have pullets that often skip the molt their first winter and continue laying. I don’t think it will hurt them to continue feeding Layer, but if you are concerned you can switch to Grower, which has essentially the same stuff as Layer except the excess calcium, and offer oyster shell on the side. Some people do that all the time and never feed Layer at all. I practically always have young chicks in the flock so I seldom feed Layer anyway, just Starter or Grower with oyster shell on the side.
I’m a little surprised to see the 16 weeks on there. The usual recommendation is 20 weeks or when they start laying, whichever comes first. That’s the other side of the coin. They don’t need the excess calcium until they start to lay, though the recommendation is to start it as soon as the first starts to lay. Some may not start for quite a while later. That’s part of why I’m not real concerned about feeding Layer to non-laying adults.
Some of the commercial egg producing hybrids will often start to lay as early as 16 weeks. It sounds like those recommendations are more for commercial operations with those specialized laying hybrids than for backyard flocks with regular breeds.
I think you will be OK to start that Layer at 16 weeks, but you will also be OK to buy another bag of Grower and offer oyster shell on the side if any start to lay, then switch to that Layer when the Grower runs out.