I texted my friend that raises them and she said 5 months was about the time for hers.
If you still have them on starter/grower, I'd keep it that way until they do lay, then switch to layer as layer feed has way more calcium and usually less protein than their grower feed does. It's getting that time of year for our chickens to slow down or stop or go into a molt even, but seems odd that by now yours wouldn't have at least laid a few before taking a winter break, but maybe they sensed it.
Are you sure they're not hiding them somewhere?
Pretty birds!