Dietary guidelines for dietary fiber are very simple to follow with a couple rules of thumb, and even though the numbers may vary according the different associations, it is pretty clear that one should consume at least around 25g/day of fiber. We believe in giving the facts, rather than commenting on them, so we'll list the different suggested values by several medical/diet associations.
Specifically, the ADA has recommended a minimum of 20 to 35 g/day for adults, and the number should be adjusted for caloric intake (2000kcal corresponding to 25g of fiber).
Another foundation, namely the British Nutrition Foundation, recommends instead a minimum of 12-25g/day, for adults.
For children, the ADA explains a very easy rule of thumb: a child should eat his age in years + 5 grams of fiber, per day. So for example if the child is 10 years old, he should consume 10 + 5 = 15 g of fiber a day.
Other values are given by the National Academy of Sciences' Institute of Medicine, recommending 38g/day of fiber for men younger than 50, and 30g/day for those 50 and older. For women, the values are slightly lower at 25g/day (younger than 50) or 21g/day (older than 50).
Scientific evidence for the elderly and diseased is still insufficient or inconclusive, so it is advised that patients with gastroenteric disorders such as pain and constipation consult their physician.
Dietary fiber hasn't been shown to aggravate symptoms in these patients, but since they are often prescribed with opioids that slow the normal digestion, this could lead to increased constipation.