When does an infant's urine become impure, and does the ruling on its impurity differ between a male and a female?

Human urine is impure and must be purified from. However, a concession has been made for the urine of an unweaned boy who has not yet eaten solid food, for which mere sprinkling (nadh) suffices. The proof for this is what Umm Qays bint Mihsan (may Allah be pleased with her) narrated, that the Prophet (peace be upon him) sprinkled the urine of a small unweaned boy on his garment without washing it. And what Ali ibn Abi Talib (may Allah be pleased with him) narrated, that the Prophet (peace be upon him) said regarding the urine of a suckling boy: "The urine of a boy is sprinkled, and the urine of a girl is washed." The difference between the urine of a boy and a girl is that for a boy's urine, sprinkling is sufficient, whereas a girl's urine must be washed. The ruling of sprinkling for a boy's urine applies as long as he has not eaten solid food and taken nourishment from it. If he begins to be nourished by food, his urine must be washed, just like a girl's urine.