My yorkie/poodle mix came into heat for the first time at 13 months, so I would say it is not unusual that yours has not come into heat yet. A couple of other small breeds at the Dog Obedience school where we went were also around 1 year.
Others have yorkies that have come into heat as 6 months so it really just depends.
Your vet can help give you some guidance- mine likes to wait until after 1 year.
My personal vote is to spay after the first heat and after 1 year. Along with my vet's recommendation, here's an article that helped me decide:
http://www.caninesports.com/EarlySpayConsiderations.pdf
You can also google the works cited in this report to get more info.
Good luck with whatever you decide.