Note: all my answers here pertain to the Latin Rite only. I am sure there are others who will want to share an Eastern perspective on fasting.
The universal rule for Roman Catholics, ages 14 and up, is to abstain from meat on Ash Wednesday and all Fridays throughout the year. This is in Canon Law, 1249-1253. This is described as a penitential practice.
In the United States and some other countries, the bishops’ conferences have given permission to substitute some other penitential practice instead of abstaining from meat, outside of Lent. During Lent, you still must abstain. You may choose to abstain all year round if that is your preferred penance. You may choose to do something else entirely, such as pray the rosary, or donate $20 to a pro-life cause, for example.
During Lent, we are called to prayer, fasting and almsgiving, in preparation for the Paschal Mystery of Christ’s resurrection. Abstaining from meat is part of our fasting practice. You may certainly do more fasting if you feel called to practice it, but the minimum obligations are outlined in those canons. Many people in my culture decide to “give something up” for Lent, this is another aspect of fasting, and not compulsory.
If you honestly forget to abstain and accidentally eat meat on one day, or you gave something up and mistakenly take it, there is no sin because the three requirements were not met (grave matter, full knowledge of the sin and full consent of the will.)
The Latin Church does not have any restrictions on drinking alcohol during the year, except that drinking to intoxication can be a sin and should always be avoided. When considering a drink on a Friday you should ask yourself if it fits in with your penitential practices and follow your own conscience.
