Is the decline of testosterone in males seen with aging actually due to senescence, or is something more sinister simply worsening over time?

This is entirely sinister. It is not a normal accompaniment of aging alone, until there is an end-stage where metabolism slows precipitously and the body stops making many hormones in preparation for the coming transition. Until then, there should be maintenance across the board of everything in life that normally takes place. To be sure, there are some cycles related to the aging process that have their particular tempo. There is not the same imperative amongst the elderly to procreate, as is true for the teenager just getting started with life, needing to find a mate and do other things of a bold nature, to make their mark and establish themselves in a profession or other career to make a good living and be a good prospect for starting a family, in order to be attractive to most females. The Extraterrestrial Alliance has seen to it that your environment is literally poisonous, with many adulterants in foodstuffs, packaging materials that leach chemicals into beverages, and alterations energetically from the environment that are injurious to the body from electromagnetic fields, and so on. All these deleterious influences conspire to cause this decline. You can add to that, as well, some genetic tinkering. All of human society is undergoing a slow-motion annihilation because that is the dark agenda underway to undercut you so you will eventually die out.