My two female Muscovies started flying at 12 weeks of age. One never gained proficiency (to put it nicely), so she decided it was safer to stick to the ground and she now only uses her wings to get up to her perch and down again. My pied Muscovy is fully capable of flight. However, she is my only flight-capable duck. Last year, she was startled by something and took off deep into the woods. I followed her from tree top to tree top for hours, then sadly gave up to go get ready for work. As I was getting ready to leave, she re-appeared at the duckio. I was lucky!
The only reason I do not clip her wings is that my ducks no longer free-range (supervised walks only) and otherwise stay in a covered run or in the barn. She’s also so used to having the ability to fly if in danger that I don’t think I could take that away from her at 4+ years of age. She’s also very bonded to her three flightless sisters.
If my situation was different (safer for free-ranging, more flight-capable ducks) I would definitely clip wings!