Sami started at 8 weeks
we were at my daughters friends house in san diego and she has a little girl yorkie( 9mo) we are planning to get them togeather as soon as sami is at least a year old, anyway he was attempting to hump her while we were there, she is a little thing I think she is around 5lbs? once we got home it stopped, until now (6mo) well he just started to do some serious hummping with his hump buddy! Im not sure about age as sami did have the the urge at 8 weeks. I thought it was strange and funny, I never had a little boy so I found it comical at that time and amazed.