I went to a RunningUSA conference a few years ago, and the medical director of the Twin Cities Marathon in Minneapolis had a session on that very question. He reported that after reviewing all the collected data from the decades of his race, and reviewing the studies that were done, he concluded that there is no empirical data that supports any age limit. In his marathon, for example, he was shocked to find that the youngest successful participant in the full marathon had been 7 YEARS OLD!
The reason he cited for his conclusion is that there is simply so much variability in physical and emotional maturity among kids that it is impossible to generalize and set a boundary. For simplicity (and perhaps liability) reasons, most races set an age limit of 16 or 18, but that’s not based on scientific data. In fact, he reported that anecdotally, he saw that kids seemed to have a lower incidence of running injuries than adults. He attributed this in part to the natural circuit-breaker that most kids have: when it stops being fun (like when they feel an injury) they stop running. Most adults aren’t that smart.
The take-away is that each kind who wants to do a marathon has to be evaluated on a case-by-case basis, with an adult reviewing how balanced and well-adjusted the student is in the various facets of life. Is he or she getting good grades? Involved in social groups? Has good family relationships and friendships? And it’s good to ask why they want to run a marathon anyway.