You see? Provided you're not a believer of some faith or another, there's no apparent reason for why a given type of animal tends to live as long as it does (in absolute terms, rather than relative to other species). Thus, arbitrary lifespan.
I'm... faintly confused by what you mean by "why", here. We know fairly well both the mechanism (various bits of biology) and the reason they're like that (evolution, if not an exact mapping of the history). It's... not arbitrary. Lifespans are a direct results of the species history. The why is "that's what their surroundings made of them"... more or less.
That we know why is that, I agree, it's fairly well described. But...
We do not know the mechanism at all. There's approximately a zillion theories, several of which don't even claim to be mutually exclusive as to the cause of limited lifespans. New discoveries are made every other day.
If anything, the situation is more like our knowledge on, say, autism: we know several different *factors* that influence it, but we've only got the pieces of the puzzle, not the big picture.
This is present ubiquitously in youth. The body slowly loses it and production of it lessens. It is used in tissue repairs and heightened levels can be used to detect certain types of cancer. Its presence has been repeatedly linked with smooth skin (not getting age wrinkles) and injections of it have been shown/linked through experiments to repair surrounding tissues.
I also heard on the radio that they are trying to use a simpler version of the compound to get the human body to create it natively.
You're cherrypicking the descriptions, probably unwittingly and/or second-hand. It's a skin component (among others); it repairs surrounding tissues, but only some particular tissues - not adding this criterion is misleading.
That the production of various enzymes and proteins drops is no news - in fact, the reason for hair greying is a drop in the production of, I don't recall off the top of my head, peroxidase? Either way, a Reactive Oxygen Species-degrading enzyme, making hair essentially bleach itself from within due to the - normally suppressed - ROS not being suppressed as effectively. But it doesn't answer the underlying question.