I would speculate that there's some physiological component involved in spellcasting ability that grows with age, in much the same way that older children are often more coordinated and stronger than younger children. I have no evidence to back this up other than the repeated mentions of 'age matters with spells', however.
It is practice. Why is, pardon, why was Hermione so much better in spells then others? Because of practice. Other children do not practice as much as Harry, or as much as Hermione did... It was somewhat suggested in part where Harry and Draco talk about muggle-borns, pure-bloods and magic.
New chapter!
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 102.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: