Here's the law from the Church's official legal documents . . .
From the Code of Canon Law:
Can. 1249 The divine law binds all the Christian faithful to do penance each in his or her own way. In order for all to be united among themselves by some common observance of penance, however, penitential days are prescribed on which the Christian faithful devote themselves in a special way to prayer, perform works of piety and charity, and deny themselves by fulfilling their own obligations more faithfully and especially by observing fast and abstinence, according to the norm of the following canons.
Can. 1250 The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Can. 1251 Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Can. 1252 The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance.
Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast.
From the apostolic constitution Paenitemini by Pope Paul VI:
III. 1. The law of abstinence forbids the use of meat, but not of eggs, the products of milk or condiments made of animal fat.
2. The law of fasting allows only one full meal a day, but does not prohibit taking some food in the morning and evening, observing—as far as quantity and quality are concerned—approved local custom.
1. In the U.S. the conference of bishops has removed the requirement to abstain outside of Ash Wednesday, Good Friday, and the Fridays of Lent.
2. Having completed your fourteenth year means that you've had your fourteenth birthday (your first year is the year before your first birthday).
3. Having attained your majority means that you've turned 18.
4. The beginning of your sixtieth year is your fifty-ninth birthday.
5. The law of abstinence thus binds everyone who has turned 14 and hasn't yet died and left the jurisdiction of the earthly church.
6. The law of fast binds those from 18 to 58 years old.
7. The laws of fast and abstinence do not bind those who have a medical condition that would materially interfere with their performance. Medical conditions that would interfere with fasting are fairly easy to think of (e.g., type 1 diabetes; people whose doctors have put them on a special diet that requires a certain number of calories or that requires ignoring calories). It is harder to think of conditions that would nullify the requirement to abstain, though, since protein is available from so many sources other than meat.
8. Beverages, even calorie-laden beverages (milk, OJ, coffee with cream, protein shakes) do not violate the law of fast. "Food" means solids food, not drinks (which count as "drink"), though disproportionate consumption of caloric beverages violates the spirit of the fast.
9. Non-nutritive or non-digestible things taken to curb hunger (e.g., water, dietary fiber) do not violate the law of fast. Medicine also does not violate the fast. The fast is from food (solid nourishment; technically, solid macronutrients), not other things (water, other beverages, fiber, medicine, vitamins).
10. You often hear the law of fast summarized this way: "You can have one full meal plus two smaller meals as long as they do not add up to a second meal." THIS IS FALSE. The law (from Paenitemini, quoted above) doesn't say anything about what the two smaller portions of food add up to. What the law says that you can have "some food" twice, and "some food" is clearly less than a "full meal," but it doesn't say anything about how much the two instances of "some food" add up to.
Obviously, the less the "some food" amounts to, the more in keeping with the spirit of fast it is, but the law does not require or encourage people to scruple over how much "two smaller meals" add up to. That's dumb, anyway, since people do not generally eat three, equally large meals (in terms of calories or volume), making it impractical to try adding up the two lesser quantities of food.
A more helpful way of thinking of it (and a way more in keeping with the way the law is written) is to think of one full meal and two snacks, a snack being something less than a meal.
11. All the above applies to Catholics who are members of the Latin
Church. Members of other churches sui iuris (e.g., Maronites,
Chaldeans, etc.) have their own law in these areas (which is what "sui
iuris" means in Latin).
LET THE ANNUAL LENT FIGHT BEGIN!