Heartworm can be given to puppies, but I'll admit that I do/did wait. Here is a heartworm info thread:
http://www.yorkietalk.com/forums/yor...-overview.html
But also, isn't it still really cold there? Many people who have a winter stop the heartworm anyway for the cold months, so it would seem you have time regardless...? I'd confirm all of that with a vet you trust.