Billy Ray Cyrus is blowing off the controversy over his then 16-year-old daughter Miley Cyrus grinding on a 44-year-old man -- saying, "It's what people her age do."

We spoke to Billy moments ago about the video that TMZ first posted yesterday -- Miley dirty dancing with Adam Shankman during the wrap party for her movie "The Last Song."

In our poll, an overwhelming majority said the dance was inappropriate ... and 40 percent of readers blamed Miley's parents -- who were not there when the dancing deed was done.

Papa Billy says Miley was just "having fun."

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