I understand. As a hobby, you can go wherever you'd like in terms of imagination with your story. As a career, you have to appeal to the audience (unless you know you have a secure source of income elsewhere). How do you appeal to the mass? You make the story funny, cool, relatable, etc. Who mostly reads these kinds of stories? High school kids.
If it was a seinen or jousei, those appeal to college women and men and older, so those cahracters are often young, somewhat average men and women. If it was a hentai, those appeal to older men so the characters are often older, average men. Yuri, yaoi, Horror and psychological are slightly different.
The age of the characters and the settings somewhat represent the audience.
EDIT: Also, for beginners and amateurs, think of archetypes and cliche settings as starting points.