I think it's genetic. Some yorkies just seem to grow hair MUCH slower than others. ZoE has very slow growing hair (...and of course, I love a long coat...
).
ZoE is 1yr & 8 months and her coat is still short. My last yorkie, Chelsea, had a floor length coat by 1 year old. I trimmed Chelsea at every other bathtime to keep her hair off the floor and neat. I hardly ever have to trim anything on ZoE...just her ears & feet.