In The Final Problem Sherlock realized he had forgotten his childhood and his memories of Eurus and what not. If you go back to the very beginning of series one that is dropped right away in The Great Game. Think about it.
SHERLOCK: Oh, you meant “spectacularly ignorant” in a nice way(!) Look, it doesn’t matter to me who’s Prime Minister ... JOHN: I know ... SHERLOCK: ... or who’s sleeping with who ... JOHN: Whether the Earth goes round the Sun ... SHERLOCK: Not that again. It’s not important. JOHN: Not impor... JOHN: It’s primary school stuff. How can you not know that? SHERLOCK: Well, if I ever did, I’ve deleted it. JOHN: “Deleted it”?
Primary School stuff. Sherlock was quite young when all of the events that followed which he had “forgotten” or in this case deleted, meaning that at that age Sherlock would have probably been learning about the solar system at that time. This means that Moffat and Gatiss planned this in the first series in 2010. Let that sink in.