* He's a seventh year: This indicates he's at least 16 years old, as Hogwarts students start at age 11.
* He's the captain of the Gryffindor Quidditch team: This implies he's a senior player with significant experience, making him likely closer to 17 than 16.
* He's older than Harry: Harry is 11 in *Harry Potter and the Sorcerer's Stone*, and Oliver is clearly a few years older.
Therefore, it's safe to assume Oliver Wood was 17 years old in *Harry Potter and the Sorcerer's Stone*.