From what age must a girl begin dressing modesty?

Some Poskim[1] rule that starting from age three, a girl is required to dress modestly, and it is thus forbidden for a man to Daven or learn Torah in her presence from age three and onwards, if she is not modestly dressed. Other Poskim[2] however rule the necessity of dressing modestly and the prohibition of learning Torah and Davening in her presence does not begin at age three but rather at a later age, when the girl’s body is developed enough to cause men to have attraction to them [i.e. age 6-7[3]]. Nevertheless, even in their opinion, it is proper to educate a girl to dress modestly from age three.[4] Practically, the Rebbe[5] concludes like the former opinion that according to Halacha girls must be educated from age three in the laws of modesty.

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[1] So rule regarding a man praying or learning in presence of a three year old girl who is not modestly dressed: Biur Halacha 75/1 “Tefach Megula” in name of Shulchan Shlomo; Ketzos Hashulchan Vol 1, Haaros Page 90 in name of Chayeh Adam [that from 3 years old a girl is considered an Ervah regarding Shema and Torah learning]; Shut Daas Sofer 1/11; Ben Ish Chaiy Bo 8 brought in Kaf Hachaim 75/10 [concludes to be stringent when possible]; Halichos Shlomo 20/9 in name of Rav SZ”A; Az Nidbaru 13/1; Likkutei Sichos [18 p 448] and other prominent Rabbis of this generation; See letter of Rav Farkash in “The Jewel of our Crown” page 79; Chesed Lealafim 75/8, brought in Kaf Hachaim 75/10, leaves this matter in question regarding any Ketana; Ashel Avraham of Butchach 75 brings sides to be stringent and lenient in this matter. See Piskeiy Teshuvos 75/8 footnote 82aa

[2] So rule regarding a man praying in view of a young girl not properly dressed: Yifei Laleiv 75/2 [brings up question and leans to be lenient]; Maharam Brisk 2/70; Chelek Levi 36; Salmas Chaim 76; The Chazon Ish [16/9] famously ruled that the laws of modesty for a girl do not begin from age 3 but rather from a later age from when their bodies are developed enough to cause men to have desire for them; Chesed Lealafim 75/8, brought in Kaf Hachaim 75/10, leaves this matter in question regarding any Ketana; Ashel Avraham of Butchach 75 brings sides to be stringent and lenient in this matter. See Piskeiy Teshuvos 75/8 footnote 81;

[3] See Az Nidbaru 12/49; Or Letziyon 2/6-12; Piskeiy Teshuvos 75/8 footnote 87

[4] Salmas Chaim 67; Orchos Rabbeinu 2/204 that this applies even according to Chazon Ish ibid

[5] The Rebbe in a letter in Likutei Sichos [18 p 448] writes that according to Shulchan Aruch the age of Tznius needs to begin at three years old and a day, although one should not be oppressive about this and should speak in a pleasant way. [For a translation of the letter see “Beautiful within” page 39]