I got Elvis when he was 2 1/2 he had just received his first pup cut.
Scoobers was 5 months old when I got him (he's a Schnorkie) he got his first grooming right after his 1 year birthday.
Princess Monster my miniature schnauzer was 9 weeks when she received her first schnauzer trim for the pre-adoption pics.
So I cannot speak from experience with a Yorkie pup but I will give my opinion, 6 months, they should have seen the groomer at least once it's pat of training.
Did you mean just a grooming or a hair cut? You are brushing at home right?
Grooming should also consist of but not be limited to:
nail trim
anal gland cleaned
teeth brushing
bath
drying
brushing
trimming or cut
Hope that helped! 