I had to learn Roman numerals as a child, but then I went to a Roman Catholic school. The system isn't taught anywhere now, is it? (Unless maybe you take Latin, which is a bit hard to find these days. I was in the first class in my high school that only had to take three years of it, not four. Did you know that you can sing all 28,000 lines of Vergil's Aeneid to the tune of The Stars and Stripes Forever and the whole thing scans correctly? But I digress.)
So if the numeral system stuck in my brain correctly, the tattoo on this young woman's spine says 1966. I can guarantee you that she is not 50 years old. The meaning is a mystery.