Teacup Yorkie how old are they till they fully grown?

theres prolly at maximum growth around 6 months, and just because her "parents" were petite or kind of large for the yorkie breed doesnt mean that she will be.. (the may have been petite, but she may grow to be pretty big by comparison
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There is no such thing as a "teacup Yorkie". They come in one size and anyone who is advertising them as "teacups" is only putting a name on the runt of the litter. They are usually very sickly and have extensive medical problems.

Runts usually stay a bit smaller than the standard, but not always does the runt of the litter stay the runt. Toy breeds reach full grown between the ages of 9 months to a year. Large breeds can grow up to 2 years.
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There is no such thing as a teacup yorkie. A 2-4lbs yorkie is just a yorkie, end of story. The standard says no more than 7lbs that's it.

Small breeds tend to reach full height at age one or just shortly after. Large to giant breed reach full height and well between 2 and 3 years of age.
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All the advice about the "teacup" label the others have given you is true. My Yorkie is about 7lbs and is very healthy and happy, good natured. I have seen other tiny ones at the groomers' and their owners said they have health issues and are not good with children. I am glad I got a healthy dog. If you read the AKC website standards for Yorkshire Terriers, they aren't even supposed to breed a female unless she is 5lbs or over.
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All dogs reach their adult size at around 1 year.

For the record, there is no such thing as a "teacup" Yorkie. It's a term used by unethical breeders to describe dogs bred to be extremely small with no regard to the overall health of the dogs. These are the types of breeders that you should avoid at all costs!
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