I remember all the basics on chicken raising but it has been 40 years, we got our first egg yesterday, only pigeon size but it was a fully formed egg. The girls are only 16 weeks, isn't that a little early?
I've had a few (very few) dual purpose pullets whose parents were hatchery dual purpose breeds lay their first egg at 16 weeks. These were not production reds or sex links, just regular dual purpose breeds like yours. Out of the remaining 21 you might get a few to start in the next month but I'd expect most to wait until after 20 weeks. But with living animals anything can happen.
The way I feed mine is to use a low calcium feed like Grower and offer oyster shell on the side. Do not mix the oyster sell in with the feed. That way the ones that need it for egg shells can eat to while the others do not eat enough to harm themselves.