Nothing magic happens at age 16. The age limit is more a statistical rule-of-thumb in that by that time the person is more likely to have a certain degree of discretion, strength, etc. required to race. The margin of error is wide as the proverbial barn door: We’ve all raced against “adults” who behave like children and we’ve all seen 14-year-olds who are better in every way than most of the adults on the track.
If a younger person is exceptional and the race director is satisfied, I have no objections, but it should be on a person-by-person basis.