Genetics are an important factor. Limes and kumquats are reported to be precocious. Grapefruit and Sweet Orange take longer,with Mandarins being intermediate. Citrumelos, despite being Grapefruit hybrids can be precocious. By virtue of close planting within a cold frame, my plants responded by quickly growing into tall plants. This seemed to promote early flowering, but only affected some of the seedlings.
A height of 3 meters seems to trigger a percentage of the trees to flower. It's also possible that having the tree tops hit the polyethylene film ceiling, then deflecting to the side accelerated the transition to maturity. The peak of the ceiling is 3.2m in height.