Early STEM Education Impact I'm looking to find studies that examine when STEM education is most critical (e.g. what years of early STEM education have a significant impact on college/uni-level students, as measured by their IQs/GPAs/SATs or any combination thereof), and what demographics (age and/or sex) of students become disengaged from STEM education. 

Results of STEM education at the pre-K and kindergarten level are of acute interest. Preferably the latest peer-reviewed studies, from North American universities of good repute.

A jumping-off point perhaps: STEM Starts Early: Grounding Science, Technology, Engineering, and Math Education in Early Childhood, and 
More Than a Foundation: Young Children Are Capable STEM Learners
Unfortunately, answers to these questions are not yet available.  As Morgan et al (2016) put it:


  ... whether and to what extent science achievement gaps display
  stable, cumulative, or compensatory growth trajectories are unknown.
  This has been due to the lack of large-scale, multivariate, and
  longitudinal studies, particularly those following diverse samples
  across the elementary and middle school grades.


Available data (eg. the ECLS) typically follows children only as far as middle school, so effects on post-secondary level students are hard to come by.

That said, as the Elisabeth McClure reports listed in the question assert, the available data suggests that early STEM education has significant impact on academic achievement and cognitive skills later in life.  The earlier the intervention, the greater the impact down the road, particularly for pre-kindergarten (eg, Duncan et al, 2007; Sackes et al, 2010; Tippett &amp; Milford, 2017), and early kindergarten (eg, Sarama et al, 2012) exposure.  That is, even where achievement gaps may be compensated for with later remedial programs, the sooner children are exposed to STEM education, the more favourable the future effects.

Correspondingly, interest in STEM education and careers is already set well before high school - from Sadler et al (2012):


  The key factor predicting STEM career interest at the end of high
  school was interest at the start of high school. ... During the high
  school years, the percentage of males interested in a STEM career
  remained stable (from 39.5 to 39.7), whereas for females it declined
  from 15.7 to 12.7.