In the movie, Newt reveals that the beast attacking New York was actually an Obscurus, which he explains is

a burst of dark power that manifests in witch or wizard that represses their magical power.

Now, he goes on to explain that

The oldest known Obscurus was 10 years old, but they're usually younger.

If this is true, why then

did Harry Potter or Hermione Granger not become Obscuri? They were not even aware magic existed until they were 11. My only thought would be that they didn't actively repress their magic, just didn't know what it was.

I don't know if there was any more detail given in a companion novel, Pottermore or the book itself, but on the wiki it made no mention of details other than was given in the movie. Are there any additional rules that would explain why this doesn't happen more often?