"The idea of overtaking Slytherin in the house championship was wonderful, no one had done it for seven years..." (PS13)
This statement refers to school years, not actual years. The 1983-84 school year is seven school years before that statement was made in the fall of 1991 (and yes, from the context we can be almost 100% sure that it's talking about the Quidditch Cup, not the House Cup). Charlie HAD to have been on the team this year, since "Gryffindor hadn't won the Quidditch Cup since the legendary Charlie Weasley...had been seeker." However, according to Rowling's age statement for Charlie, he wasn't, as he was only a first year at the time. This is an error in the text.