Demi Moore was about the age of a high school senior. The pictures appeared in the March 1981 edition of Penthouse Spain. She was born in November of 1962, so the very oldest she could have been is barely 18. Given the lag times in magazine publishing, she may even have been 17 during the shoot.
In fact, many of these same images appeared in a Oui magazine which was dated January, 1981, so she was almost certainly 17 at the time of the photography.