Ke Jie won 8 out of 10 when went against Lee though. Lee is probably not the strongest in the world right now.
Ke Jie is 19 years old. Lee has been a pro for nearly 20 years. Lee is not old but certainly not young. Go game prodigies seem to peak when early 20's, much like mathematicians.
My personal hypothesis for the reason why Magnus Carlsen is the youngest chess champion of all time is that with the rise of online battles and computer battles, the age where the blend of crystalized intelligence and working intelligence combines lowers.
This has obvious implications for the recent rise of machines immediately correcting human mistakes in mathematics and physics.