Ash Wednesday and Good Friday are obligatory days of fasting and abstinence for Catholics. In addition, Fridays during Lent are obligatory days of abstinence.
For members of the Catholic Church, the norms on fasting are obligatory from age 18 until age 59.
When fasting, a person is permitted to eat one full meal, as well as two smaller meals that together are not equal to a full meal. The norms concerning abstinence from meat are binding upon members of the Catholic Church from age 14 onwards.
If possible, the fast on Good Friday is continued until the Easter Vigil (on Holy Saturday night) as the "paschal fast" to honor the suffering and death of the Lord Jesus and to prepare ourselves to share more fully and to celebrate more readily his Resurrection.
Those who are excused from fast or abstinence, besides those outside the age limits, include those of unsound mind, the sick, the frail, pregnant or nursing women according to need for meat or nourishment, manual laborers according to need, guests at a meal who cannot excuse themselves without giving great offense or causing enmity and other situations of moral or physical impossibility to observe the penitential discipline.
Below is a short video on why we fast.