Many people scoff at how old the actors for the Potter films are getting. Well, they don't even know the half of it. I have photographic evidence that Harry Potter is not actually a 15 year old boy in Order of the Phoenix, as has been proposed, but in reality, a graduate student.
As you can see in this screenshot, Harry Potter is dressed in a manner that is quite similar to your average male graduate student. His tan blazer is very tightly fitting, with a slight boxy effect to the shoulders. The tee shirt underneath suggests that he is trying to connect with the undergraduates that he TAs for, but it is tight to show off that he is more than a big brain. You can see from the looks on the faces of those around him, he is discussing some important theorem or another with the undergraduates he teaches. Ron is clearly only listening because it may be on the test, Hermione is trying eagerly to sleep with him by engaging him in his witty dialog and Neville is trying to comprehend what is going on, but as a freshman, is at a loss for what his TA, Harry, is saying. I would suggest that Harry is probably only getting his masters degree, because if he was aiming for his Ph d, there would be elbow patches on his jacket.