While it's possible it was something like finally getting proper nutrition or that something delayed puberty, or some random circumstance that caused his pituitary to start working again, like one too many hilaaaaarious slaps on the head, it's probably more likely the historical record was simply inaccurate/falsified/mythologised, and he reached his full height more or less at adulthood. After all, there were likely no really unimpeachable records of his earlier height.