You are dealing with living animals so the word "never" does not apply. You are also dealing with individual animals so about anything can happen. Chickens or breeds may have tendencies but a lot don't read the book. Many individuals do their own thing despite what the book says.
I don't have bantams and I don't know when your breeds of bantams would "normally" start to lay. 18 weeks does sound pretty young for that even with full sized dual purpose fowl. Even in spring or summer, I would not expect yours to lay for another month, but that can vary a lot for each individual.
It is fairly normal for pullets that start laying in the late summer or fall skip the molt their first winter and keep laying until the following fall. It's really unusual for older hens to skip the molt when the days get shorter. It's the days getting shorter that triggers the molt, not the length of days, by the way.
Last year I had some mixed breed full sized pullets reach laying age in early December. About half started laying just before the winter solstice, during the shortest days of the year and while the days were still getting shorter. That's the age my flock usually starts to lay if they reach laying age in summer. The others were all laying by February. I do think some delayed laying because of the short days-long nights but it did not phase some of them. I don't use lights to extend the day.
From my experience, no it is not true that they absolutely, without a shadow of a doubt, forever and forever, always and always, anywhere in the northern hemisphere won't lay until next year if they haven't started by now. It is possible they won't lay until next year and the days getting shorter may affect them, but it is also very possible you will get some eggs by Halloween. Even in the middle of summer it is possible some won't lay until they are 9 or 10 months old or even older.
