16week and 21week layer feed switch?

You can/could go either way. Feed everyone a Flock Raiser type All Flock product such as Grower and offer a calcium supplement on the side. Or, your 16 week old birds are probably past the danger of being exposed to heavy doses of calcium for too long a period for their renal health. It's close.

Me? I'd buy a bag of Grower and feed that for 4-6 more weeks. But, that's just me. My thinking is that I just cannot say with certainty that my 16 week old pullets won't wait until week 26 to start. We've got some strains that wait until week 36 to start. I wouldn't want them on Layer for months and months before laying.

It's a close call. Do what you think is best.