usually it is done when they get fixed, spayed or neutered... and it is very important to get it done, yorkies are prone to tartar and dental problems due to tartar and retained baby teeth is just one more contributing factor.
It is very common for yorkies to have retained baby teeth and nothing unusual.
It will be unusual however if your vet makes you put him/her under anesthesia 2 times instead of pulling it out during the spay/neuter.
Just get it done, you will never regret it !!!!
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