I guess I don't consider them adult until they are of the age to lay eggs - more like 15 - 20 weeks. They wouldn't need layer food until then and it's not balanced for growth. Yes, they are probably full frame at 7 weeks, being a small breed, but not mature. My large breeds are not fully mature until they are about a year old. I don't know if my ducklings from this spring will lay until next spring and I will keep them on flock raiser until they show signs that is near, or start laying. That said, they have free choice oyster shell calcium every day.
That was my consideration. I buy my flock raiser in pellets and put my 2 - 3 week old ducklings on pellets. They get a non medicated chick starter with brewer's (or nutritional) yeast for added niacin before that. They have access to dirt, etc., from about 2 weeks on, so they don't need grit.