Pouya Ariakhah scored 21 points for Iran, while Klim Ryukhov led Kazakhstan with 13 points.
Iran will play Puerto Rico on Friday.
The 2025 FIVB Volleyball Men's U-21 World Championship is the 23rd edition of the FIVB Volleyball Men's U-21 World Championship, contested by the men's national teams under the age of 21 of the members of the Fédération Internationale de Volleyball (FIVB), the sport's global governing body.
It is taking place in China from August 21 to 31, 2025.
Iran is the defending champion.