Charlie Weasley left Hogwarts to study in Romania at some point in his sixth year (1990-1991), after having played at least one Quidditch match as the Gryffindor Seeker.
Rowling's statements about the ages of the eldest Weasley boys seem to be inconsistent. However, careful analysis strongly suggests that Charlie must have left Hogwarts during his sixth year to go study dragons in Romania. The Lexicon uses this logic to determine the dates on Bill and Charlie's timelines because it's the only way to rectify all the various canon statements.
The essay "Quidditch Through the (Weasleys’) Ages or, The Unusual Career of Charles Weasley" contains an interesting and detailed analysis of birthdates and Hogwarts attendance.
From the Web
Pottermore feature: Charlie Weasley
Harry Potter Wiki: Charles Weasley