I think esturk meant that AlphaGo is barely beatable now, so by the time anyone else gets a chance, it will have improved in the meantime and even a stronger human player won't be able to beat it.
By that measure, Fan Hui beat AlphaGo before Lee Sedol did, since he was playing an earlier and bit weaker version of more or less the same complete, distributed, system.
Lee is 3rd ranked as of now. He was 1st for almost 10 years before that.
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.