"In" vs. "among"

The sentence is referring to a pathology that affects individuals, where among is group oriented: 1. surrounded by somebody/something; in the middle of somebody/something: A house among the trees; They strolled among the crowds. 2. being included or happening in groups of things or people: A British woman was among the survivors; He was among the last to leave. 3. used when you are dividing or choosing something, and three or more people or things are involved: They divided the money up among their three children.

In the sentence cited in the question the nosebleed sufferers are related only in a statistical sense. If the sentence was discussing a meeting of an epistaxis support group then an individual nosebleeder could be said to be "among" a group where many of the members were over 50 years old and suffered frequent nosebleeds and in some, the attacks could be quite serious.

Yes, among would be better than in in a medical textbook.

In medical writing, among is the standard term when reporting on, or even when merely referring to, a category or group — of patients, of candidly-observed people, clinical-experiment subjects, laboratory animals, . . .

The term in is used more for specifying a location, as in:

Lesions in the sinus lining are also common, but sometimes serious, among people over age 50.

An example of among in the title of a recent medical article:

“Global, national burden of diseases, injuries among children & adolescents.”
Institute for Health Metrics and Evaluation News, 1/27/2016.

Countless other examples can be found with searches such as:

http://www.mdlinx.com/otolaryngology/search-articles.cfm/among

[Sorry if the above web sites limit the number of free searches or views, but you get the idea.]