1. Hogwarts isn't even real. Sorry, but it isn't. 2. Within the story, seeing and entering Hogwarts isn't a matter of believing in it. It's down to having magic. There are spells preventing Muggles from entering it. 3. Every character has been 11 upon first entering Hogwarts, या 12 if his या her birthday would be on the 1st of September. There are no examples mentioned of students starting later for some reason.
आप do realize that Hogwarts was created द्वारा J. K Rowling? For her books? It's not real. It's fiction. That's all it is. So, basically, no matter how hard आप believe, it will never happen. And people believing it will happen really need to have a big dose of reality and their heads examined.