Just to add a little bit on the Mary Sue thing:
The problem with these types of characters isn´t necessarily the fact that they are better at something, or even special, but rather the reason behind this. Often, these types of characters are made because people creating them want to have someone special, who is above everyone else and is loved by everyone because of how amazing they are. They want their characters to be center of the story and end up causing everyone around them to dislike the character.
Do you want to make a really young, yet pretty capable mage who started studying magic as a little kid, but because they spent literally their whole life in Dalaran, don´t know anything about the world except what they read in books and have absolutely no skills besides magic? That´s great, because it allows you to RP interesting scenarios, especially in situations where you can´t rely on the magic and suddenly, you are absolutely useless.
Do you want to make a really young, yet pretty capable mage because you are going to be awesome young wizard like Harry Potter (or more like Hermione Granger if she was better at Defence Against Dark Arts) and you will deal with every problem and will be awesome? You are doing it wrong.