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.

Brazilian Portuguese
Narrator: "Tai Kamiya, 5th grade."

Tai: "My name is Tai Kamiya. I'm a student at Odaiba School's 5th grade. Sora is also in 5th grade."

Tai: "He is Matt, also in 5th grade."

Joe: "Joe Kido, 6th grade."

T.K.: "Takeru. Takeru Takaishi, 2nd grade."

Joe: "Yeah, Mimi. She's from 4th grade."

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

Digimon Adventure: Our War Game!
Sora had a birthday recently. --Replace with quotes from the movie, both the Japanese and English versions. Other dubs are also valid, but every version must have a different header.--

Senri Hori has her birthday party during the entire battle.

Digimon Adventure: Storyboard

 * Miyako: "Somewhere in Japan, a girl (in third grade) watching with her older sister (in middle school) as she types on the keyboard." An arrow pointing to the text reads "the girl in glasses that will appear in series 2".
 * Monks: "Young monks-in-training (about 13 years old) in Tibet."

Toei Animation official site

 * Taichi Yagami: Elementary school, 5th grade
 * Sora Takenouchi: Elementary school, 5th grade
 * Yamato Ishida: Elementary school, 5th grade
 * Kōshirō Izumi: Elementary school, 4th grade
 * Mimi Tachikawa: Elementary school, 4th grade, "Kōshirō's classmate and class 1's beauty"
 * Jo Kido: Elementary school, 6th grade
 * Takeru Takaishi: Elementary school, 2nd grade
 * Hikari Yagami: Elementary school, 2nd grade

Unknown publication

 * Taichi (7 years old)
 * Hikari (4 years old)

Digimon Adventure: Official Encyclopedia

 * Taichi Yagami: "Odaiba Elementary School, 5th grade"
 * Yamato Ishida: "Same age as Taichi [...]"
 * Sora Takenouchi: "Taichi's classmate."
 * Kōshirō Izumi: "Odaiba Elementary School, 4th grade"
 * Mimi Tachikawa: "Kōshirō's classmate."
 * Jo Kido: "Odaiba Elementary School, 6th grade"
 * Takeru Takaishi: "[...] 2nd grader in elementary school."
 * Hikari Yagami: "Taichi's younger sister and a 2nd grader in elementary school."

Digimon Adventure: Official Encyclopedia II

 * Taichi Yagami: "Taichi is a 5th grader in elementary school."
 * Yamato Ishida: "The combination of Yamato, a 5th grader in elementary school, and Gabumon!"
 * Sora Takenouchi: "Sora, owner of the Crest of Love, is a 5th grader in elementary school, and Taichi's classmate!"
 * Mimi Tachikawa: "A girl at 4th grade in elementary school, she has the Crest of Purity!"
 * Koshiro Izumi: "Koshiro is a 4th grader in elementary school, and is the same age as Mimi [...]"
 * Jo Kido: "A 6th grader in elementary school, he has the crest of Honesty!"
 * Takeru Takaishi: "Takeru is Yamato's younger brother, and is a 2nd grader in elementary school who has the Crest of Hope!"
 * Hikari Yagami: "Hikari, Taichi's younger sister, is a 2nd grader in elementary school."

Digimon Adventure (Movie) comic

 * Taichi Yagami: "A 1st grader in elementary school who lives in Odaiba."
 * Hikari Yagami: "Taichi's 3 years old sister."

Digimon Adventure: Character File

 * Taichi from Hikarigaoka, 7 years old: "Taichi when he was in Hikarigaoka, 4 years ago."

Digimon Series Memorial Book: Digimon Animation Chronicle

 * Taichi Yagami: "A 5th grade student at Odaiba Elementary School [...]"
 * Yamato Ishida: "A 5th grade student at Odaiba Elementary School [...]"
 * Sora Takenouchi: "A 5th grade student at Odaiba Elementary School [...]"
 * Kōshirō Izumi: "A 4th grade student at Odaiba Elementary School [...]"
 * Mimi Tachikawa: "A 4th grade student at Odaiba Elementary School [...]"
 * Jo Kido: "A 6th grade student at Odaiba Elementary School [...]"
 * Takeru Takaishi: "A 2nd grade student at Kawada Elementary School [...]"
 * Hikari Yagami: "Taichi's younger sister and a 2nd grade student at Odaiba Elementary School."

Taichi's family

 * Taichi (8 years old): "Taichi 3 years ago [...]"
 * Hikari (5 years old): "Hikari 3 years ago [...]"

Mimi's classmates
"4th grade students who were taking part in the camp."
 * Micchan, Tāko

Digimon Adventure (Movie)

 * Taichi Yagami: "1st grade student at elementary school [...]"
 * Hikari Yagami: "Taichi's 3 years old sister."

Digimon: The Official Character Guide

 * "T.K." Takeru Takaishi: "But he's only eight years old, so I guess it's natural."

Digimon: Digital Monsters - The Official First Season

 * TAI / Taichi Yagami: 10
 * MATT / Yamato Ishida: 10
 * IZZY / Koshiro Izumi: 9
 * MIMI / Mimi Tachikawa: 9
 * JOE / Jo Kido: 11
 * T.K. / Takeru Takaishi: 7
 * KARI / Hikari Yagami: 9