Flat-coated Retriever?

At what age do they normally have their full adult coats?

Actually it varies with the dog. Generally, they all grow out of their puppy coats at about 8 - 10 months of age (the soft velvety puppy coat becomes less velvety). After that, they develop their rich thick coats at different rates. One of my boys had his full coat at about a year and a half (seems about average for my dogs). One of my girls didn't really get a full, thick coat until after 2-1/2 years (she's a slow developer). In my experience, most of the girls are slower than the boys in developing their full coat - usually after the 2nd or 3rd heat cycle. Spayed and neutered FCRs are known to develop very thick, flyaway coats (my dogs can attest to that) rather quickly. Usually feathering of the mane and on the legs appears first. The best indication of a full adult coat is thick feathering on the underside and on the tail.

Most of the time (not always) males have thicker, longer coats than the girls. Liver FCRs tend to have thinner coats than the black ones (again not always)

Hope this helps.

usually about a year, sometimes longer though

I'm not 100% sure about Flat coated Retrievers but With my Golden you dont see their real full coats until at least 18 months old.

I'm hoping Gin Bail see's this question he has Flat Coated Retrievers. He can tell you much better than I.