Note that they'll be more familiar with nursing jargon than, say, a recent nursing school graduate. But you can see how this would help you widen your base from just nursing publications to college newspapers on campuses which feature nursing programs. Once you know age and occupation, this question becomes easier to answer. Again, you can settle on a range. This will especially help you determine your vocabulary.
Some publications tell writers to assume readers have no more than an eighth grade education.
A highly educated audience may also be more tolerant of controversy, have greater interest in global affairs, or have a broader perspective on historic repetition. Start with the world and narrow this down. If you're writing in English, that immediately reduces your audience. This can help with vocabulary and level of understanding. Also consider the type of structure your reader occupies.