I planted an avocado seed 16 years ago and this year is the first year for it to have flowers. The flowers open and within a few days they are gone. No fruit. Across from this tree is another tree belonging to my neighbor which was planted from seed about 5 years ago. His tree had fruit last year but this year he had the same problem, flowers and five fruits – but as soon as they were about pea size, they fell off. These two trees are about 20 feet apart.
It can take 8 to 20 years for a seed planted Avocado to produce fruit.
You may also need a pollinator.
Here are some articles with more information.