Albus was only eighteen, but his parents died. He has not only a younger brother, but also a stupid sister. In order to take care of his brothers and sisters, the young genius had to give up his ideal and return to Godric Valley.
Grindelwald is younger, only sixteen years old. He also showed amazing talent, but because of his "terrible experiment", the German Institute of Demstrom had to expel him, so Grindelwald had to start his own travel career.
In the midsummer of Godric Valley, two talented teenagers met.
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At that time, Albus was just a little boy who had never seen the world, and his knowledge was far less than that of Grindelwald. Gellert's wonderful experience deeply attracted Albus, and his theory made Dumbledore fall into it like a moth.
It was during this period that the two men set their future goals and put forward the slogan: "For greater interests." Later, all the movements of the dark Lord Grindelwald were practicing this.
Dumbledore was young and ignorant, and was instigated by Grindelwald to step into a completely opposite road to his future life. If not, Grindelwald may have succeeded. With Dumbledore's help, he will completely change the magical world, and Voldemort will have nothing.