You don't have much option. We almost always just omit the subject and just use honorific in conversation. To attract the person's attention before attempting to talk to him/her, you can say "저기요", meaning "excuse me" in English. If you really have to address the person, say "그쪽분께서" but try to sound respectful, for this is also used in aggressive situations. "아저씨" is a term that might offend some men in their 30s. Use it for at least those over 40. "아줌마" is more offensive than "아저씨" for women regardless of their age. "총각", literally "bachelor", is only used by an old female to address a man who is a bachelor. I don't think "청년" is used to address a person in conversation. "학생" is used to address a person who seems to be in their 10s and 20s. But you have to be older than them or have a higher social position. For example, a college student may call a highschool student who is 2 years younger "학생" but a college student wouldn't call another college student "학생" even if the latter is 5 years younger, for example. But that is unless the former is a TA and the latter is one of his/her students.

아저씨 can be used for young man or aged man above your age. If you call a young man who is younger than you, you can call like this 학생 청년 총각

I think "저기요" is the most common expression to call a stranger no matter the person you try to call is older or younger than you. it's close to "Look." in English.