Zakat is obligatory on the young ones if there are older ones with them, and if they are all young, then according to Abu Hanifa and Muhammad, their zakat is not obligatory. However, according to Abu Yusuf, their zakat is obligatory, and one of them should be paid. Abu Huraira (may Allah be pleased with him) said: "When the Messenger of Allah (peace be upon him) passed away, and Abu Bakr (may Allah be pleased with him) fought against those who denied the zakat among the Arabs, Umar (may Allah be pleased with him) said: How can you fight the people? The Messenger of Allah (peace be upon him) said: I was commanded to fight the people until they say there is no god but Allah. Whoever says it has protected his wealth and himself from me, except for his rights, and his account is with Allah. He said: By Allah, I will fight those who differentiate between prayer and zakat, for zakat is the right of wealth. By Allah, if they were to withhold a young female goat that they used to give to the Messenger of Allah (peace be upon him), I would fight them for withholding it. Umar (may Allah be pleased with him) said: By Allah, it was only that Allah, the Glorious and Exalted, opened the heart of Abu Bakr (may Allah be pleased with him), and I knew it was the truth." This is in Sahih al-Bukhari 2: 507. The young female goat is the female offspring of a goat that has not yet reached one year of age. And from Sufyan ibn Abdullah (may Allah be pleased with him): "Umar ibn al-Khattab (may Allah be pleased with him) sent him as a tax collector, and he was counting the people’s young goats. They said: Are you counting us with the young goats and not taking anything from them? When he returned to Umar ibn al-Khattab (may Allah be pleased with him), he mentioned that to him. Umar (may Allah be pleased with him) said: Yes, you count them with the young goat that the shepherd carries and do not take it." This is in Al-Muwatta 1: 265.