I wonder what other difficult competitive endeavor someone can take up at 20 and have the potential to reach the <1%.
bb_gum: I keep saying "Chigorin!" and your side keeps ignoring it.
Unless the accounts are false that Chigorin only took up chess seriously at 24, he managed to challenge Steinitz twice for the World Championship and provided a decent showing both times.
Like I said to this argument earlier, just because we can't draw a line, doesn't mean it doesn't exist.
However, that doesn't mean the line gets drawn at GM.
I will concede that a 20 year-old can't become World Champion. All the stars have to line up for that including a mammoth appetite for work as pfren points out.
But World Champion is being the Numero Uno player in the entire world.
GM is "just" being among the top 1400 players in the world, which is a significantly lower bar.