i haven't read much about it, so here's my conjecture based on what i know about bacteria. babies are 'sterile' in utero and have to pick up bacteria on their way out. it's possible that in the early years, they are better suited to establishing new colonies of bacteria, because we rely on bacteria for a host of functions (like generating vitamin k) and need to get them set up quickly, so maybe the exposure as a baby is more important to the adult colonies. it's possible that it's related to the amount of exposure, so that the constant exposure as a baby has more impact than the occasional spin the bottle as a teenager. we're also not all the same in terms of our hosting abilities -- not every mouth is as hospitable an environment (inherently, not just hygiene-wise) and so some people are more prone to occupation by hostile bacteria than others. nutrition (in terms of access to micronutrients) probably plays a role (more so than the amt of sugar), as does overall environment, including environmental toxins (that might interfere with the formation of enamel), quality of water (hard/soft/added fluoride), etc.

all conjecture!