The deadline is August 31st. You get your invitation when you are eleven. No invitation, no Hogwarts.
Never mind. I hope you do not take it as criticism. As you said, Snape took him earlier. I would do the same. I liked this story, so don't take this the wrong way, please.
Here we have the system deadline August 31st, but you can get exception if "you are prepared to go to school" and you are born before the end of December.
Ha ha I just don't think there is a place that says that in the books. You can get your letter if you 'will' be that age by a certain deadline.
Thanks again for the reviews.