Age Six:
Tzitzis: A child is obligated to wear a Tallis Katan from the age of six and onwards.[1] Regarding children that have reached the age of Chinuch, which is from age six until the age of thirteen, one may be lenient to follow those opinions[2] that rule the minimum measurement of a Tallis is a total of one Ama in width and one Ama in length.[3]
Cheder – Talmud Torah:[4] In previous times, the son would not be brought to Cheder until he was six-seven years of age. He would be homeschooled by his father until this age. From when he can talk he would be taught verses of Torah by heart until age four. [Immediately[5]] after turning three, he would be taught the letters of the Alphabet.[6] At age four he would be taught how to read and he would continue to practice reading the written Torah until age 6-7, when he would be brought to the Milameid.[7] In today’s time’s, however, this system has changed, and the custom is for a son to be brought to Cheder at a much earlier age, as explained next.
[1] Siddur Admur “A child is considered to have reached the age of Chinuch for the Mitzvah of Tzitzis from 6 years old and onwards.”; Lev Chaim 101; Ben Ish Chaiy Lech Lecha 14; Kaf Hachaim 17:10
Ruling of Admur in Shulchan Aruch and other Poskim: In 16:1 Admur rules that a child is considered to reach the age of Chinuch for Tzitzis from the age of 9. This follows the ruling of the M”A 16:1 and so rules M”B 16:9; See Bach; Sefer Hachinuch 386; Ridbaz 2:2
[2] Peri Haaretz 1 brought in Birkeiy Yosef 16:1; Chesed Lealafim 16:5; Many Poskim in Kaf Hachaim 16:2 agreed to this opinion; See Haaros Ubiurim 779; See also Beir Heiytiv 16:1 in name of Peri Haaretz who brings an opinion that holds of an even lesser Shiur; Piskeiy Teshuvos 17:2 writes the Shiur is to be a minimum of 24 by 24 cm on each side.
[3] Siddur Admur; Piskeiy Teshuvos 17:2 writes the Shiur is to be a minimum of 24 by 24 cm on each side for a 6 year old, however it should be 36 x 36 cm for a nine year old.
[4] Rokeiach 296; Seder Chinuch Hayiladim of Rabbeinu Efraim Mibuna; Seder Chinuch Hayeladim Sefer Hasufos 282; Shevach Habris p. 138; Shulchan Menachem 5:34; Hearos Ubiurim Ohalei Torah 1019 p. 44; Hiskashrus 904 p. 13
[5] Rama Y.D. 245:8; Igros Kodesh 12:288
[6] Admur Hilchos Talmud Torah 1:1; See Rama 245:8 “Immediately after reaching age three, he is to be taught the letters of the Torah”; See Igros Kodesh 12:288; 14:39, printed in Shulchan Menachem 5:33; Nitei Gavriel 19:1
[7] Admur Hilchos Talmud Torah 1:1; Michaber 245:5