Digimon Frontier/Appendices/Ages and grades

Episode 01
Shinya's birthday.

Japanese
Takuya: "He [Tomoki] must only be in third grade or so..."

Takuya: "I'm in fifth grade! I'm older than him. Izumi: "So we are the same age? Come on... I thought you were younger than me." Takuya: "I'll be 11 years old soon!" Izumi: "On which month?" Takuya: "August." Izumi: "So I'm older after all. My birthday is in May, so I'm three months older."

Japanese
Takuya: "You [Tomoki] can't. You're only in third grade, aren't you?"
 * Given that Tomoki doesn't refute this claim, we can assume he's in third grade.

Junpei: "Well, I'm in sixth grade!"