The age at which a baby boy’s urine must be washed off
Question

When does the urine of an infant become najis (impure?) Is there a difference between boys and girls?.
Praise be to Allah.Human urine is impure and must be cleaned off, whether it comes from a child or an adult, male or female, except it is less serious in the case of a baby boy who is not yet eating food, in which case sprinkling water is sufficient to purify it. Al-Bukhaari (223) and Muslim (287) narrated from Umm Qays bint Mihsan (may Allah be pleased with her) that she brought a son of hers who was not yet eating food to the Messenger of Allah (peace and blessings of Allah be upon him). The Messenger of Allah (peace and blessings of Allah be upon him) sat him on his lap, and he urinated on his garment, so he called for some water and sprinkled it, but did not wash it. 
Al-Tirmidhi (610) and Ibn Maajah (525) narrated from ‘Ali ibn Abi Taalib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said concerning the urine of an infant boy: “Sprinkle the urine of a boy and wash the urine of a girl.” Qataadah said: This applies so long as they are not eating food; if they are eating food then it must be washed in both cases. Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 
This hadeeth indicates that there is a difference between the urine of boys and girls. In the case of boys’ urine it is sufficient to sprinkle it, but the urine of a girl must be washed. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Sprinkling means putting water on it without rubbing or squeezing, until it covers the entire area. 
If it is said, What is the reason why the urine of a boy who is not eating food is sprinkled and not washed like the urine of a girl? The answer is that the reason is that this is what is enjoined in the Sunnah, and that is sufficient reason. Hence when ‘Aa’ishah was asked why a woman has to make up fasts that were missed because of menstruation but she does not have to make up prayers missed for the same reason, she said, “That used to happen to us at the time of the Messenger (peace and blessings of Allah be upon him), and we were commanded to make up the fasts and we were not commanded to make up the prayers.” 
End quote, from al-Sharh al-Mumti’, 1/372 
With regard to the age of the boy whose urine may be cleaned by sprinkling, we have quoted above the words of Qataadah: This applies so long as they are not eating food; if they are eating food then it must be washed in both cases. What is meant by this is when the child wants food, is nourished by it and asks for it, is does not refer to when he eats whatever is placed in his mouth. 
Ibn al-Qayyim (may Allah have mercy on him) said: Rather the ruling on sprinkling ceases to apply when (the child) eats food and wants it and is nourished by it. End quote from Tuhfat al-Mawdood bi Ahkaam il-Mawlood, p. 190 
Shaykh Muhammad ibn Ibraaheem (may Allah have mercy on him) said: It does not mean that he sucks and swallows what is placed in his mouth, rather it is when he wants food and eats it and shows an interest in it and asks for it, or he shouts or points to it.  This is when he is regarded as eating food. End quote from his Majmoo’ Fataawa, 2/95.