When do the rulings on an adolescent girl begin to apply to a little girl?
Question

With regard to little girls, when should we not greet them with salaam, and start to lower our gaze and not look at them – from what age?.
Praise be to Allah.The scholars are unanimously agreed that it is haram to look at a little girl or shake hands with her if that is accompanied by desire and pleasure. But if there is no desire, then some of the fuqaha’ stated a guideline of when the girl reaches an age at which she would normally become desirable. When she reaches this age, it becomes haram to look at her and shake hands with her. If she has not reached an age at which she has become desirable, then there is nothing wrong with shaking hands with her and looking at her. This is the view of the Maalikis, and is the more correct view according to the Shaafa’is. 
The Hanafis and Hanbalis set as a guideline the age of ten years. They said that when the little girl reaches the age of ten, her ‘awrah becomes like that of a free adult woman, so it is not permissible to look at her or touch her or shake hands with her. 
It says in al-Mawsoo’ah al-Fiqhiyyah al-Kuwaitiyyah (31/52): 
The Hanafis are of the view that up to the age of ten, the ‘awrah of the young boy or young girl is that which is regarded as the extreme ‘awrah of the adult, and after that age the ‘awrah is the same as an adult’s. 
The Maalikis narrated that for a girl who is seen as desirable, her ‘awrah is the same as that of an adult woman with regard to looking and washing. 
The more correct view according to the Shaafa’is is that it is permissible to look at a little girl who is not seen as desirable, apart from the private part, which it is not permissible to look at. 
The Hanbalis said: if the small child is female and ten years old, then her ‘awrah with regard to non-mahram men is her entire body. End quote. 
See: Haashiyat Ibn ‘Aabideen (1/407, 408); al-Bahr al-Raa’iq by Ibn Nujaym al-Hanafi (1/285); Mughni al-Muhtaaj (3/130); Haashiyat al-‘Adawi (1/185, 336); Kashshaaf al-Qinaa’ (1/266); al-Mughni (7/462). 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) favoured the view that she comes under the same ruling as an adult woman when she reaches an age at which she becomes desirable, even if she has not reached the age of ten. 
He (may Allah have mercy on him) said: There is no ruling on the ‘awrah of a little girl, and she does not have to cover her face, neck, hands and feet, and the little girl should not be obliged to do that, but when the girl reaches an age where men may be attracted to her and desire her, then she should observe hijab so as to ward off fitnah and evil. That varies from one woman to another; some of them may develop quickly and look mature, and others may be the opposite. End quote. 
See also the answer to question no. 20475 and 43485. 
And Allah knows best.