How many calories does a old person need per day?
It depends on the individual. It's the same thing as asking how many calories a young person needs each day - there's no definite answer.
The number of calories marathon runners eat per day while training varies from person to person. Both women and men need to eat 2,000 to 2,500 calories per day plus an additional 100 calories for each mile they run. They also need to be sure that roughly 65 percent of their calories are coming from carbohydrates, especially complex carbohydrates.