It is not grammatically correct. If it is a question as you wonder how he did you should say:

Comment ce garçon de quinze ans a-t-il résolu un Rubik’s Cube en seulement six secondes ?

If it is a declaration, like the title of something that will explain how the kid did it:

Comment le garçon de quinze ans résolut un Rubik’s Cube en seulement six secondes.

However you will often see it as you have written on the Internet, mostly because journalists do not care about correct grammar. So if you want a parody of these titles your phrase is what you need.