Your vet is right, Goldens can grow until they are 18 months old. When Carina came home my vet couldn't believe she was six months old because she was rather small. Now she's 10 years old and 70 lbs. Sizes can vary a lot, my first Golden, also a female was very large, 90 lbs., my second was smaller, 75/80 lbs. Pups grow at their own pace.
In any case, what does it matter? A Golden is just as sweet, loving and smart no matter the size.
Please give Daisy a big hug from this other Golden mom.