I definitely would wait until he is completely out of his destructive puppy stage. Nine hours in an empty house is a LONG time for a bored Yorkie.. you never know what you might come home to!
It's safer not only for your possessions but also your baby to stay confined when you aren't around. I would wait until
at least a year and a half, maybe two years. I know someone who waited until 3 years. I also know people who have never confined their Yorkies.
We leave 10 month old Franklin and 3 year old Maggie home alone unsupervised only when we will be gone less than an hour. I think it also depends on your Yorkie's temperament. Is he normally calm or does he get rambunctious at times?