I have done some at 10 to 16 weeks. Depending on how well they are filling out. I wish I had some now. I love duck meat. I am incubating 7 goose eggs, so I hope I can have a similar taste in a few months. They should fill out fairly young.
My buddy who raises pekins for meat tells me that the commerical industry butchers at exactly 7wks 2 days because the pins feather will clean up better. I've had decent luck between 10-12 weeks. After week 12, those darn pin feathers start showing up again. For my own table, I prefer them closer to 6 months