Well, I agree in part (I'm sure there is ONE 13-year-old somewhere who does; if there is, it's statistically insignificant) but I don't necessarily agree that intellectual capacity is the same thing. Intellectual capacity is fairly easily measured, it isn't correlatable with emotional capacity, and emotional capacity isn't entirely a learned thing. We know the brain is still developing, and that perceptions of the world change significantly between the ages of say 12 and 22, 25 (depending on gender).
This is why even for intellectually superior students, they don't recommend moving up too fast - a grade or two - because there isn't a corresponding emotional maturity.
But that he may or may not be wrong, the fact that you and I are having this debate, this discussion, is why he shouldn't be silenced, as some are calling for.