In "Chamber of Secrets", Draco Malfoy mentions that the chamber was opened 50 years prior, and that it was before his father's time [at Hogwarts], and we know that Voldemort was the one to open it.
In the "Half Blood Prince", when inside Dumbledore's memories, we see Tom Riddle (A.K.A. Voldemort) sorted in Slytherin, and it is mentioned that Lucius Malfoy is the Prefect. That would make Lucius older than Voldemort, and since the chamber cannot have been opened before that point in time, then it couldn't have been before Lucius' time.
Is there something I am missing, or is it a continuity error the author missed?