18 mth old hens are not laying and I have no idea why.

First questions are, are you sure they are not hiding eggs somewhere, or might there be something that is eating/stealing eggs.
http://edis.ifas.ufl.edu/ps029 is a good page on possible reasons. First thing i usually do is check to be sure they don't have mites or something, them try upping the protein % a little if they are on 16% layer. Normally you would expect them to go into molt around the age they are now, yours might have gone into molt early for some reason, they do take awhile to get back in stride.

 

14-18 months is prime time for chickens to molt. The decreasing day length also will have a major effect on hens of this age. Even if they laid well last fall, without extra lighting to make 14 hours of light, they may not lay very well. Broodiness could also have an effect on the rest of your flock. I started with a broody Welsummer about 6 weeks ago & even with light, my hens are only laying 2-4/12 each day. The Welsummers are molting after less than a year of laying, which is really poor! First time in 20 years that I've had chickens molting so early. Don't know if it's crappy chickens or the weird weather we had this year. Hope this helps.