Twitter's official policy is that an account becomes inactive after 6 months - at that point, they reserve the right to release the account (in practice they rarely do this, though - there isn't an automated job releasing inactive accounts or anything)

https://support.twitter.com/articles/15362-inactive-account-...

@N (now @N_is_stolen)'s last post was 4 months ago, so he is still technically considered an active user.