Does a yorkie's hair have to be so long as an adult?

You can keep the hair any length you like.
References :
http://friskypups.com

doesnt have to be long unless u show them
References :

You can keep the hair *any* length you want it.
References :
Mutt

You can keep your yorkie in any length of hair you want as long as you keep it maintained. The majority keep their yorkies clipped short since it is a lot of work to upkeep the full coat. Take a good pic of what length & clip you like to groomer & they will groom your dog as long as his coat is in good shape & not matted. It will take regular grooming appts to maintain the look.
References :
30yr of grooming

An adult Yorkie's hair WILL grow as long as in the first picture, but unless you plan to show the dog (that's a show clip in the pic), there's no reason to keep it long unless you want to.

The second pic is a Yorkie in a puppy clip, and many Yorkie owners keep their dogs in that style. It's adorable and MUCH easier to care for than the long, silky hair (which mats and tangles easily).

But if you want to keep a Yorkie in a short clip, you'll have to have it groomed pretty often (or learn to do it yourself, but it's not easy).
References :

You can do whatever you want with the fur. It's your dog. Just don't dye it or do anything stupid with it.
References :

Unless your dog is a show dog or you have the time to brush and de-tangle all that hair I suggest you cut it. The hair will stay short if you keep getting it cut, otherwise it will grow long again.
References :

I keep my male yorkie in a puppy cut. So I take him to get a hair cut every 4 months or so.

He's in the pic
<————-
References :

duh just keep taking him to the groomer to keep it short or clip it yourself just like if you dont cut your hair it will grow long..same way.
References :

If they are not for showing then you can have their fur cut to whatever length you like. I prefer short hair myself so that you can see which end their eyes are.
References :

i think the hair has to be long for shows but you can keep it short if you wish
References :

If you plan on showing, it must be long.

If its just a pet tho, you can trim the hair to whatever length you want.
References :

i keep my yorkies short, you can keep it however you want it.
and she isn't groomed that often.
References :