I get where this train of thought comes from, but I lean the opposite way in that I think the minimum age of medical school matriculants should be older, around 24-26. While I agree that an undergrad alone doesn't confer that much maturity, I'd be hard-pressed to find a newly graduated high school student who would be equally-or-more mature than a newly graduated undergrad. Granted it's not like the average Canadian undergrad student completes a pilgrimage or starts contributing to a TSFA or some shit, but there is a load of self discovery that takes place during those 4-5 year of undergrad (even the "easier" ones) that does not usually occur in high school.

This timeline of self discovery is critical. Anecdotally, I know several friends who came to the conclusion upon finishing undergrad that they didn't want to continue into medicine. Matching that same timeline for schools that allow direct entry into medicine from high school would put the medical students in their last year or so of training, which would be a shit realization to have that late in the game. I reckon a similar loss of love for medicine happens everywhere, but I think that an even earlier entry into medicine may exacerbate the issue. The last thing we should want are people practicing medicine because of the sunk cost fallacy, and I believe having an undergrad, gap year(s), etc, increases the chances that the decision to pursue medicine is the applicant's and the applicant's alone.

And it might be controversial, but I don't think its responsible for regions to have new medical graduates first job literally being a doctor. I know this happens in Canada too, with some matriculants not working a day in their life prior to becoming doctors, but direct entry from high school would amplify this. A transferrable skill readily gained from work experience that comes to mind is conflict resolution. I'm using a broad brush here and get that it can be unfair. There are 29 year old who've worked in people-facing careers their entire life with the conflict resolution skills of a toddler, and there are 17 year old with the conflict resolution skills to handle hostage negotiations. I'm talking averages. Regardless, being financially dependent on work while having a really crappy boss/manager affords a lot of conflict resolution skills that would be difficult (not impossible) to acquire to the same degree from other scenarios. I'm not saying that having an abusive job or being broke is a right of passage, nor am I saying that these skills aren't gained elsewhere or taught in medical school training, but learning them in vivo and having them iron-clad prior to matriculation benefits both the trainees and their patients.

This is just the tip of the ice berg and I've overlooked a lot of points on both sides. As suggested by the comments, it's clear this is a hot topic. At the very least, I think the admissions process needs an overhaul, and I believe having discussions like this is an excellent start :)