Minimum age to appear fully nude sexually?

How old must an actress be to appear fully nude in sexual scenes (and performing sexual actions) in a movie?

If I got it right, she was 20 when Nymphomaniac was released, which means she was probably 19 during shooting.. That's awfully young, especially to be doing such explicit sexual actions (with a girl no less, not that I'm saying it's wrong or anything, just considered taboo in social norms) and displaying close-ups of her vagina.