He is the eldest son of Molly and Arthur Weasley, who eloped only a year or so before this.
Rowling's statements about the ages of the older Weasely children seem to be contradictory. If correct, Charlie would have been a Seventh Year student when Ron was a First Year. However, careful analysis of all the fact given by Rowling strongly suggests that Charlie actually left school in his sixth year to go study dragons in Romania. The Lexicon therefore dates Bill and Charlie's births based on that logic, the only logic which satisfies Rowling's statements.