That seems to be the only thing that makes sense, but does the text say that anywhere?
It does not. It doesn’t quite not say it, either:
1) At 15, Voldemort creates his first Horcrux from Abagail Myrtle.
2) After he “grasp[s] the stupidity of ordinary people”, Voldemort decides to invent a better ritual.
3) He spends “years” refining it in his imagination.
4) Quirrel finds one of the Horcruxes which Voldemort had hidden in the “hopeless idiocy of [his] youth”
So, is “when he grasped the stupidity of ordinary people” + “years” < “youth”? It seems unlikely. But I do not think that it is quite ruled out.
That seems to be the only thing that makes sense, but does the text say that anywhere?
It does not. It doesn’t quite not say it, either:
1) At 15, Voldemort creates his first Horcrux from Abagail Myrtle.
2) After he “grasp[s] the stupidity of ordinary people”, Voldemort decides to invent a better ritual.
3) He spends “years” refining it in his imagination.
4) Quirrel finds one of the Horcruxes which Voldemort had hidden in the “hopeless idiocy of [his] youth”
So, is “when he grasped the stupidity of ordinary people” + “years” < “youth”? It seems unlikely. But I do not think that it is quite ruled out.