Don't blame it on the age of the breed. Blame it on the fact that size is polygenetic. It's not just one simple gene that determines size. It's several genes, plus some more, plus outside influence. We also add nutrition and gender into that equation, and it's a crap shoot.
We know that given identical sets of genes, a male dog will grow 1 inch taller than a female dog. So really, any bitch over 15 1/8 inches tall is the genetic equivalent of an oversized male, so do we remove them from our breeding programs as well? How about a 13 7/8 inch male, that is equivalent to a 12 7/8 inch bitch?
Removing over/undersized dogs from a breeding program in no way guarantees only having insize dogs. It's way more complicated than that. And quite frankly, size (and to some extent, color) is the last thing we should worry about. How about we produce healthy, physically and mentally sound, happy dogs that look like Shelties? How about producing dogs with good fronts who can run agility into their teenage years? How about producing dogs that can actually work a farm?
I think it says a lot about our breed that we can complain about size as a major thing. That means we aren't complaining about bloat (big dogs), or seizures (Poodles, BCs), or cancer (goldens), or breathing problems (all the squishy face dogs), or major skeletal issues.
But we should not cause these problems by removing so many genes from our limited gene pool based on something as arbitrary as size.