Have there been any recent studies on the effect of age on PhD success rates in the US? I think the title is fairly self-explanatory. I am wondering if any recent studies (past 20 years or so) have been published that look at possible correlations between age and the 'success rate' of PhD candidates? I understand that 'success rate' may be somewhat subjective - I'm thinking of possible metrics such as dropout rate, years to completion, no. of papers published, impact factor 5 years after graduation.

The background to this is that I am 37 and planning to apply to Engineering PhD programs in my area this Fall. However, I had a casual conversation with someone recently who is connected to one of those schools, who commented that there might be some thinking on the part of an admissions committee that I may be 'past my peak' for their PhD program (they weren't representing it as their own personal view). However, this seems at odds with what I have read online and advice I have received from a couple of Professors that I know (not related to these schools), which indicates that more mature PhD candidates tend to do well and are quite highly regarded.

Now, I'm not trying to point fingers or accuse anyone of ageism here. For all I know, my contact's suspicion might be mistaken, or I may have misinterpreted. But, it occurs to me that it would be great if I could point to some hard data that makes a persuasive case that mature PhD candidates on average perform at least as well as fresh graduates, which might help to pre-emptively 'head off' any doubts regarding my age.
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.