I have a 10-year-old Avocado tree that is about 15 to 20 feet tall, but has never produced any fruit? I'm in Central Florida.
If it was grown from seed or a pit, it can take 15 years before it bears fruit.
Check out this article for more information, including a description of the odd pollination cycle described by Colin.
There are several possibilities: insects, weather, pollen source. Since this is a persistent, chronic problem the first two are unlikely, leaving the third. The avocado tree has very peculiar flowers; they are both male and female capable, but they are only rarely capable of self-pollination and sometimes not capable at all. The tree achieves this by opening its flowers one day as male and producing pollen for insects to collect, closing tight, and then some hours later opening again but this time ready to accept pollen. And the whole tree coordinates the opening and closing, so today it functions exclusively as a male, and tomorrow it will be a female.
So how does pollination occur so that fruit can be formed? There has to be a nearby avocado that is on a different schedule, flowers in the same calendar period and is compatible with your tree. The insects visit that tree when it is male, become covered in pollen and then come visit your tree on a day when it is female and voila the process is complete.
For a good authoritative description of this type of issue see this publication from Western Australia. No doubt your next question is "What variety is complementary to mine?" Well for that you would need to know specifically what your variety is, or use a shotgun approach by planting a group of known different varieties one of which might possibly serve. You could also look for a known variety outside your immediate area, collect some pollen yourself and hand pollinate your tree. That would be a fun exercise that would in the fullness of the season show what partner tree will pollenize your tree.