I think JY just made young GJ's cognitive development so slow to stack the odds against GJ relative to YH. Here was the underdog born and raised in a hard life compared to a smart good looking kid born in a life of luxury, yet it is the underdog that achieves greatness and becomes a (fictional) historical hero. Perhaps it's a lesson in life JY intends his readers to learn.
In real life, a child who is unable to speak until 5 is severely retarded so severely mal-nourished. I dunno if there's ever been any child in real life whose initial cognitive development was so slow but still developed into a cognitively normal human being. By ROCH, GJ should no longer be described as "stupid."