MSTP is nearly impossible even for the brightest domestic students. “prestige” Schools like Harvard/MIT only have a handful a year out of a class of almost 400. Genius consider those people geniuses.
Being non-traditional, IMO your options are:
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apply to work in a research lab as a technician, get the foot in the door and work your way up. Take community college classes for pre-req, they are cheaper and easier and looks the same on paper as any expensive post bacc program would.
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Go to med school in your country. Many may only require you to take an exam and get a certain score. Then just come over to US and take/and pass the licensing exams. You can get a PhD separate. If you don’t match, then do post doc in a lab, again to get your foot in the door.
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Some on already mentioned: get a job at a US uni in your current field, and then apply as faculty – standards are lower because you are faculty. It is also about getting foot in the door.
Age doesn’t matter. Oldest guy in my class was 49. A prior (almost) doctor from Viet Nam who never graduated and spent over 10 years in labor camp because he got caught trying to escape back in 1979. At 49 decided he still wanted to be a doctor, applied and got in.