Reddit mentions of The Making of Great Men: Male Domination and Power among the New Guinea Baruya (Cambridge Studies in Social and Cultural Anthropology)

dehydratedpink and timdiggerm are right, I explained briefly above.

I don't know exactly how the question of impotence relates with the semen. It might, seeing how semen is a male lifeforce that make you "strong" but I don't recall exactly if it's part of the symbolism. On the other, I do tend to remember that speaking of bedroom woes in public is a huge no-no to them, and it's a tool sometimes used by women to shame their husbands, who on occasion will hang themselves if the woman goes too far.

Papua NG is amazingly interesting if you like to see how intricate and diverse human society can be. Even as a layman, just out of curiosity it's worth reading a couple of books. This one is really good for a general and accessible overview of the Baruya.

EDIT: Also, he is not always that much older. Sometimes the age difference is only a few years. The "uncle" thing can be deceiving because we are so used to the older uncles in our systems.