Then you realize it was this guy: https://en.wikipedia.org/wiki/Erik_Demaine
Then you realize it was this guy: https://en.wikipedia.org/wiki/Erik_Demaine
I understand the idea behind that phrasing but I'm not sure I agree with it. Are you no longer a child prodigy once you turn 18? I don't think I'd ever say "former intelligent child".. Would I?
Somewhere along the line you convert from child prodigy to genius assuming you maintained your ability above the rest of the pack.
As an example, take Dijkstra's algorithm. It's far from the only thing that he is known for and I am by no means trying to diminish his accomplishments but how many people really tried to solve that problem before him?
I might be subject to some kind of fallacy or bias but I feel like if I'd never heard of Dijkstra and was presented with the task to find the shortest path between two nodes in a graph I could have come up with that algorithm. Maybe not in the first day but eventually at least.
It's that whole "standing on the shoulders of Giants" thing. The giants allow us to see further by learning from and building on their work, but they also picked a lot of the low hanging fruits in their career. Sure they left a lot of things undone but they probably picked out the juiciest lowest hanging fruits first and most of the ones left are either less juicy or higher up or both.
As this effect continues we end up in a situation where simply getting to the point where one can begin to push the boundaries of a specific field takes decades of learning from these giants before us, and now we're millions of people all looking for those juicy low hanging fruits while there's hardly any fruit left on the tree at all.
All that to say I think it's unfair to compare modern researchers to Einstein and similar giants. Making a revolutionary discovery like these men have done is possibly less a matter of raw intelligence and more a matter of circumstance. You need the right people in the right place with the right people around them and the funding for it and so on.
Maybe "a child prodigy in his youth" would be both precise and succinct enough, but at the same time language is for humans and I feel humans know what is meant by former child prodigy.
Also I'm saying that nobody can come up with the theory of relativity again. That big juicy fruit is picked. I'm not saying it was low hanging, but it also wasn't at the top of the tree. Maybe if someone else figured out relativity and Einstein was born 40 years ago he would just be another relatively unknown physicist today. Maybe he'd have figured out some other amazing revolutionary thing, who knows. He certainly wouldn't have been the one to figure out relativity if it had already been done.