A prime candidate for the “villian” causing men to have weaker erections as they age is endothelin-1. As noted in the wikipedia description of this compound (LINK), which increases with age:

In a healthy individual, a delicate balance between vasoconstriction and vasodilation is maintained by endothelin and other vasoconstrictors on the one hand and nitric oxide, prostacyclin and other vasodilators on the other. Overproduction of endothelin in the lungs may cause pulmonary hypertension, which can sometimes be treated by the use of an endothelin receptor antagonist, such as bosentan, sitaxentan or ambrisentan. The latter drug selectively blocks endothelin A receptors, decreasing the vasoconstrictive actions and allowing for increased beneficial effects of endothelin B stimulation, such as nitric oxide production. The precise effects of endothelin B receptor activation depends on the type of cells involved.

Hmmm. The quote refers to pulmonary hypertension but I believe the same applies to erectile dysfunction. “Endothelin receptor antagonist”…I like the sound of that.