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An avocado tree grown from seed (non-grafted) can take 7 to 10 years to bear fruit, if at all. You can help by planting more trees, i.e., grafted seedlings.
Avocado trees have functional male and female organs in each flower. However, the flower may open as a male first, then the next day open as a female, and vice versa.
This article tells more: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-with-no-fruit.htm