Mine's 5.7 (immersion test by dr. a couple months ago), and I still have a deposit of fat on my lower back above butt... I think that's the body's last reserve. I wouldn't want to go any lower than I am, as if I get sick and go without eating for a few days, it could might cause me to be malnourished more than if I had a little fat to fall back on.
To answer the OP's question, I don't think there's a straight answer, but there are various tables based on age, health, ethnicity, and lifestyle.
Just to be clear; 5.7 is very very low, and very few people believe me (until I show pics of abs with even the smallest veins showing, and individual muscle fibers visible on my delts).
Charles