How long does it take for a Golden Retriever to get full feathers and size?

I have a 20 month old Golden Retriever. I've had her since she was a puppy. She's about average size for her breed. Everywhere I go people think she's a puppy. She's got some feathering around her hind legs, tummy, fronts, and tail. How long will it take before she reaches full maturity?