Have there been any recent studies on the effect of age on PhD success rates in the US?
I don't have exact references handy right now, but a researcher at UC Davis named Dean Simonton did some work on academic and creative output and age. The upshot is that up until actual senescence--so certainly no younger than around 70 for neurologically healthy adults--the relevant variable is "career age" (the time one has been working in a particular field) and not biological age.