Response: When young boys and girls reach seven years of age they are encouraged to fast so as to become accustomed to it; And it is upon their guardians to encourage them to do so, just as encouraging them to perform their salaah, so when they attain puberty fasting becomes incumbent upon them.
If they attain puberty during the day, their fast for that day is accepted; So if a young child, who is fasting, was to turn fifteen [years of age] at noon then that day [of fasting] is accepted from him. The first part of the day would be classified as supererogatory and the latter half as obligatory, so long as he had not yet attained puberty, either by the appearance of pubic hair, or ejaculation of sexual fluids from desire. This is also the case for the girl, as the ruling for both is equal [in this matter], except that she has an extra sign indicating her having attained puberty and that is [the onset] her menses.
Shaykh Ibn Baaz
Fataawa Ramadhaan – Volume 1, Page 234, Fatwa No.178;
Tuhfatul-Ikhwaan bi-Ajwibah Muhimmah tata’allaq bi-Arkaan al-Islaam – Pages 160-161. Fatwaonline.com