Digimon Adventure/Appendices/Ages and grades

Japanese

 * Taichi Yagami: Odaiba Elementary School, 5th grade
 * Sora Takenouchi: Odaiba Elementary School, 5th grade
 * Yamato Ishida: Odaiba Elementary School, 5th grade
 * Kōshirō Izumi: Odaiba Elementary School, 4th grade
 * Mimi Tachikawa: Odaiba Elementary School, 4th grade
 * Takeru Takaishi: Kawada Elementary School, 2nd grade
 * Jo Kido: Odaiba Elementary School, 6th grade

Except for Mimi, this info is repeated when the children introduce themselves to the Digimon.

Japanese
Jo Kido: "I'm Jo Kido from the sixth grade!"

Unknown publication

 * Taichi (7 years old) - Pajamas: Design for movie Taichi
 * Hikari (4 years old) - pajamas: Design for movie Hikari

Digimon Series Memorial Book: Digimon Animation Chronicle

 * Taichi (8): Episode 48 flashback design, "Taichi 3 years ago [...]"
 * Hikari (5): Episode 48 flashback design, "Hikari 3 years ago [...]"

Movie

 * Hikari Yagami: "Taichi's sister who has become 3 years old [...]"

Digimon: Digital Monsters - The Official First Season

 * Tai / Taichi Yagami, Age 10
 * Matt / Yamato Ishida, Age 10
 * Izzy / Koshiro Izumi, Age 9
 * Mimi / Mimi Tachikawa, Age 9
 * Kari / Hikari Yagami, Age 9