At what age are shelties finished growing?
Some lines of Shelties develop very quickly. In others, they are slow to mature.
As a general rule, I tell people that Sheltie growth stops at about 10 months old and that's when you can see how tall your pup will be.
At about 18 months is when the Sheltie growth plates in the legs close (FYI: if you are planning to train in agility, young dogs need the bone in their legs to calcify before doing repetitive athletic jumping over obstacles).
It has been noted that dogs spayed / neutered early tend to be taller than those of the same breed spayed / neutered later because it delays the closing of those growth plates.
After that, your Sheltie will continue to "fill out" over the first year or two. His coat can take up to 3 years to develop into a beautiful full double coat.
Hope that answers your question. Thanks for asking!