Being a diet-and-health "guru", I can tell you that in 99+% of cases where someone is deemed "healthy" or as having a "perfect" diet (same thing in most instances, really), it is not the case, most-often not by a long shot.
So, this "healthy" individual most likely had multiple issues that hadn't surfaced yet.
A simple way to explain what I mean:
The body's priorities are to maintain an alkaline blood ph (around 7.2), keep it oxygenated, hydrated and as toxin-free as possible. It'll compensate for and take measures against all manner of "sins" without the person's having any knowledge or clue as to what's going on "under the hood".
As we see in general life, you can only compensate for something until the reserves run out and this is what happens in the body. The capacity to counter poisons and acidity, for example is not never-ending. Cells become saturated with toxins, leading to disease (dis-ease). Alkaline-buffering minerals run out if not replenished whilst being called upon for acid binding, which results in one's peeing them out daily.
Things tend to happen in slow-motion, but when the shit hits the fan, "everybody" panics at the "sudden" appearance of symptoms (we call them diseases). Diseases don't just happen, generally-speaking, but rather, they're a manifestation of underlying activity's coming to a head.
So, back to the 18-year-old. Whatever was going on internally, it'd likely have compromised the immune system for starters. Furthermore, I'd suggest that given another 6 months, a year or whatever, the issues would've manifested visibly and at that point the teenager would not have been deemed "healthy". IOW, there's a period of vulnerability between the time the body starts to suffer and when it becomes apparent on a macro, everyday level.