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Pianist who perform in theatres, or concert halls have to get paid according to the musician's Guild which, by the way, you need to be a member of and pay your dues. Age is not important once you are 18 but you still need to get in touch or the person hiring you, has to let the Guild know. They pay a fee which allows you to perform.

Yes , in some locations a person may need to be a member of the American Federation Of Musicians. An 18 year old can be paid to perform music . In a school setting where you are a student you would not be paid . There are situations where a person would need to over age 21 to perform in venues that serve alcohol .