How’s this for a character background: (This character actually exists in an on-going campaign as an NPC.) From his earliest memories, he has been an orphan and been fascinated with magic. One day at the orphanage, the local noblewoman was visiting, but a band of villains attacked her. They used magic to put everyone to sleep; everyone but this little boy. He played possum, but watched them. When the heroes arrived, he was able to give them descriptions and point them in the right direction. Because of his information, the heroes were able to rescue the noblewoman. She returned to reward him, and asked him what he wanted (expecting to give him cash). He said he wanted to be the apprentice of the wizard/enchanter among the heroes. Now he is. That was three or four years ago, and now he’s 15 and gaining a strong magical education.
So where did your character learn to cast spells? From a powerful adventuring wizard hero! My character was in the right place at the right time and through luck and slightly higher mental attributes, resisted a powerful spell.
Now what’s wrong? Well, this PC now has a very powerful mentor and guardian who will likely bail him out of trouble whenever it becomes necessary. Do the other PCs have that? Should any of them have that?
Gamemasters need to demand that players actually come up with character backgrounds, but they cannot let those backgrounds cause unbalance in the game. Too often GMs let these things go because they sound cool, not thinking of the nightmare it will cause later in the campaign.
Force the players to think up something cool, but restrict them from anything unfair. If you’re stuck for good ideas, pick up a copy of Character Foundry (e23 or RPGNow), the character building books we put out a short time ago.