I don't know about silkie's specifically, but it can be as young as 16 weeks. The average is around 20 weeks. Your guess is as good as mine as far as the longest wait.
I have Buckeyes and EE. EE started laying between 20 -24 wks. 1 Buckeye started laying yesterday
, she is 22 wks. I am expecting the other one to start laying anyday. Earlier is not usually better. The later they start to lay the bigger their eggs usually are.
My EE, MaryAnn lays an 1 3/4 oz egg about 4 days a week, she started laying at 20 wks. Her sister started laying at 24 wks and lays a 2 oz egg everyday (missed 2 days since mid January). My Buckeye's first egg yesterday was 2 ozs and in a month or so will be bigger. It's worth the wait.