Owloysius must travel to another world where he discovers he has become a human. In Canterlot High, he meets Principal Philomena and becomes a subtitute teacher. He also meets human versions of Angel Cake, Gumball, Winona, Tank and Opal.
Just because Tank was a tortoise doesn't mean he's old! Sure they have a long life span, but that's just like saying "a human can last up to 120 years, so that automatically makes him/her 120 years old."
People on Reddit often miss title and description because of the Hover Zoom addon, but yeah, when visiting the dA page, while I can understand that the art can totally steal the show ... the inquisitive mind eventually has to win text as a good friend. Whenever you want to know something, ask yourself: "What would Twilight Sparkle do?"