This scientific review article reviews over 150 original research papers about dietary protein, absorption, protein recommendations, how it changes with age, meal protein composition, and more. It's pretty jargon heavy and rather long. However, if you have an interest in health and fitness, and building muscle in particular, I recommend reading it.
People participating in resistance training may need a higher protein intake than sedentary people, which has become "common" knowledge in the fitness world.
This levels off at around 25g.
This means that saturating you system with protein or amino acids won't continuously stimulate myofibril MPS (new muscle fibers).
This article goes into the effects of aging quite a bit, mainly looking people 50+ yrs. Summary here is that the elderly need less protein — their bodies can't absorb it. High protein diets are worse for the elderly as they are more strenuous on the kidneys.