Slacker Quackers

These slacker quackers still have not laid any eggs.  They are now 6 months old (27 weeks).  We have read different ideas as to when they should commence, and had hoped that we would have eggs by now.  In Storey's Guide to Raising Ducks, David Holderread mentions in one place that they should commence between 16-20 weeks of age.  In another place he suggests 20-24 weeks.  This assumes adding artificial lighting in the fall to imitate spring.  Another resource I foun ...