A boy or girl can be addressed мальчик and девочка.
A person looking between about 14 and 35 could be addressed молодой человек (male) or девушка (female).
A more senior person lacks the proper addressing in the modern language. There are several forms of address of limited use:
гражданин / гражданка (used by law enforcement, may also be used by strangers, but this sounds too official)
товарищ (widely used in Soviet time but almost obsolete now, though still acceptable. Without adding a surname, only appropriate for males)
мужчина / женщина (are only appropriate to draw attention of a complete stranger you're unlikely to ever talk to again, in an urgent case).
Мужчина, вы кошелёк уронили.
Женщина, вы не выходите на следующей остановке?
уважаемый / уважаемая can be used (somewhat ironically) to make a polite remark to a person violating the norms of behavior:
Уважаемый, разрешите пройти.
Уважаемая, уберите тележку.
If in doubt, it would be best to use an impersonal addressing: прошу прощения, извините etc.
A child before about 8 can call an adult дядя / тётя or бабушка / дедушка. The latter are also appropriate for younger people to address senior people, but only if they called them сынок, дочка, внучек or внучка first.
An Orthodox priest, monk or a nun can be addressed батюшка / матушка.
A police officer can be addressed товарищ полицейский, or, if you can read insignia, товарищ with addition of their rank: товарищ сержант, товарищ капитан. Usually, servicemen are very sensitive to rank formalities so use the first if in doubt.
When starting a letter, the recipient can be addressed уважаемый with compulsory addition of their first name (and probably patronymic) if you know it: уважаемый Василий Львович, уважаемая Светлана. Without the name, this addressing would sound derogatory (as in the previous examples).