Masturbation
Other cultures have rites of passage into manhood that end in the first ejaculation of a male. This is often by the hands of a tribal elder.  In some tribes such as the Agta in the Philippines, stimulation of the genitals is encouraged from an early age.  At puberty, the young male is then paired off with a "wise elder" or "witch doctor". This person uses masturbation to build his ability to ejaculate in preparation for a ceremony. The ceremony ends in a public ejaculation before a celebration. The ejaculate is saved in a wad of animal skin and worn later to help conceive children.  In this and other tribes, the measure of manhood is actually associated more with the amount of ejaculate  than penis size.  Frequent ejaculation through masturbation from an early age fosters frequent ejaculation well into adulthood.