They have? Because last I checked puberty and physical growth tend to peter out around 13-14 for females and 15-16 for males (male growth plates seal at about 15; in females, about a year after menarche, and menses itself stabilizes about two years after menarche).
Does it make his nebulous conspiracy theory more or less believable that you somehow think the age at which we stop warehousing kids actually has nothing at all to do with physical maturation?