@AshlynM, “My parents often tell me” is present tense, which implies that they’re still living. If the OP had said “told me,” then I would draw the same conclusion you did.
It refers to the percentages of the style of dance which will be played during the evening. A 50–50 dance night is 50% modern dances and 50% old-fashioned. A 60–40 dance has more modern dances, thus appealing to the “younger” people.