I was at a friend's house this weekend when I noticed her avocado tree:
I was very intrigued, and was preparing to offer her all sorts of vegetable trades, when she disappointed me by telling me that it has never fruited in the 10 years since she's lived here. The tree was already here when she moved in, so she doesn't know the variety or age.
The tree looks reasonably healthy, and it's flowering right now. It's about 14' from ground to the tip.
What are the most common reasons that an avocado tree won't fruit?
This area is definitely avocado-friendly, as I have two neighbors about 1/2 mile from each other that both have heavy bearing trees.
This friend is a 3 mile crow flight from those other two trees, but it's a very similar climate.