The practice of the Church is that every Christian should regularly perform some acts of penance, this is for the good of the soul, but also for the reparation of sinners in the whole world. One form of this practice is abstinence and fasting.
As the inquirer stated, fasting, which is the reduction of the amount of food, is for those age 18-59, and abstinence, which is from meat, snacks, and extravagance, is for age 14 and up. There is no upper age limit to abstinence.
This law is generally imposed on all Fridays and Ash Wednesday. However, many places have in recent times reduced it to only Lent; and Canada and some countries have further reduced it to only Ash Wednesday and Good Friday. However, the reduction is only on removing the pain of sin from not abstaining and fasting on Fridays; we are still encouraged to practise it all year round. And this is the same with those who are not under the age range, say a 70-year-old, though not required to fast, may choose to fast as an act of penance, provided it does not jeopardise his health.
But what if a certain person, due to health reasons or otherwise, is not fit to abstain or fast? The Church allows for a number of conditions when a person is exempt from fasting, which include medical conditions and moral obligations.
So, if a person has diabetes or an elderly has complicated medical conditions, they are not obliged to abstain or fast. Yet, some form of penance should still be part of his practice, like performing an act of patience or kindness to those who care for him, or offering his suffering for the conversion of sinners.