I'm originally from the UK and, while you are correct that university at ~18 is usually the point-of-no-return, "high prestige" universities often have signific

I'm originally from the UK and, while you are correct that university at ~18 is usually the point-of-no-return, "high prestige" universities often have significant expectations on performance within a predefined set of subjects (at least within engineering) during the admissions process. It would be very challenging for someone lacking the above-average levels of interest/talent in relevant fields at the age of 16 (spurring them to take related college-level courses) to get into a prestigious STEM course - outwith extraordinary circumstance.