I apologize in advance for being so blunt as to be potentially offensive, and this post is not going to be short.
Abraham Flexner's 1912 report was interesting both for its content and motivations. Hopkins by and large pushed for it (and funded it?), he gave them glowing reviews regarding their curriculum and gave scathing or at best sub-par reviews to most of the medical schools on his report. I find his work somewhat biased in its overall effects but necessary given the quality of graduate reportedly being produced by some institutions at that time. Over half of US medical schools in existence at the time were subsequently closed as a result of his report, one of the largest concerns I recall from reading it was the necessity of an undergraduate degree before going to medical school. This is somewhat ironic given the flavor of this thread, but I think the question the OP was actually trying to posit was, 'Could an accelerated track into a medical degree specifically for the practice of surgery be viable for Vietnam and World War II combat medics who may only have been 18 or 19 years old'?
Baseball involves a *very* different skillset and while even the MLB seeks candidates with greater experience in many cases, their skillset is neither potentially lethal nor emergent as that of a surgeon. With a few exceptions, the American formula for medical education is as follows, almost always starting at age 18: 4 years of undergraduate education (age 22), 4 years of medical school education (age 26), and 3-7 years of post-graduate training (finishing at age 29-33) known as residency because the trainee physicians actually lived in the hospital. General surgery is a five year training experience, Neurosurgery is a seven year training experience, and these days both are increasingly competitive with many candidates opting for a master's degree (+2 years) or research experience (+1 - 2 years) before applying somewhere in the cycle. Fellowships add an additional 1-2 years, so the actual medical component of surgery alone from education to post-graduate training is *9 years* and can be as long as *12 years*.
Could you have someone much younger go through the system? Yes, there are programs that allow for combination BS/MD programs and graduation of medical school at age 24 and some people may start at 16 or 17, but the additional life experience of older or more diversely experienced candidates not only facilitates the establishment of rapport but also the psychological reserves of the rigors and pressures of the American medical training system. One of the reason physicians tend to congregate in their own little social groups is because this co-ed fraternity has its own language, hazing rituals, and experiences that other people often have trouble relating to. At 18 I could see a combat medic/veteran having an edge on the competition, but arguing for motor skills is only part of the question. It is learning that subtle texture, that minimal difference, that little observation on so many different illnesses that can *literally* mean a life or death difference both under the knife and not. Judgement calls and experience buffers, the opportunity to learn not only how best to perform an operation but timing and best -opportunities- to do so, these are only a sliver of the real-world education that post-graduate training provides. While an argument to shorten medical school exists, it is not popular, and the extension of residencies by a full year in the face of work-hour limitations is a hot topic. The precarious balance between seeing as much as possible while there is still oversight and getting the jobs allocated done in a timely manner is precarious, it is a pendulum not likely to stop swinging in the near-term and while an 18 year old with prior experience might have an edge on fellow students they are -not- ready for actually performing the operations needed. Time is not easily substituted or shortened in this case, and without that buffer of experience and training all the motor skills in the world will do someone no good when they are knee-deep in guts and blood only to ponder, "Now what"? without a readily available answer.
So no, I do not believe that the 'baseball model' applies nor should it be allowed to apply to surgery on the basis of dexterity or fine motor coordination.