1. At what age range are standing vertical jump numbers valid?
They are assumed to be valid for people in their early 20s.
2. Are there any curves showing the degradation of the standing vertical jump with age?
Not that I know of. And who cares anyway? You know what the curve will look like. Feel free to draw your own.
3. What test can be used to determine the inherent athletic ability of a 68 year-old old?
Why is this important to a 68-year-old? Vanity? Do you have an NFL Combine to attend? You know that your athletic ability has degraded from where it was in your 20s. And I can't think of a better way to need surgery than to start trying to assess an irrelevant parameter that involves loading the now-inelastic connective tissues that might be semi-successfully holding your ass together.
4. Does the reaction time of an individual mean anything to inherent strength ability?
Not that I've ever been able to tell. I have very fast hands and a very shitty SVJ. Always have.