Avocado-Seeded Plant

Manfred says...
I have a seedling that's eight years old and has never flowered and another four years old which had a fruit earlier this year. They are both in very similar positions.

Be patient, eventually it will flower and fruit, but it will do it at a time of its choosing, regardless of how much you input.

Don't let anyone tell you it may not ever fruit. (Cue cracked record-) fruit is fundamental to plants. The chances that it won't fruit is about the same as a child being born without legs. It can happen, but no-one warns against sexual reproduction of animals because of the likelihood.