For what purpose? Except if one is planning on competing in a physique contest going down to a REAL 5% is counterproductive.
Notice that I mention REAL 5% because most people grossly underestimate their body fat percentages. Calipers, even in the hands of a good coach normally underestimate body fat by abut 3%. At a REAL 5% you should barely be able to pinch any skin at all. In the picture above I was a tad over 7%.
Few athletes really are at 5% even though may claim it. In most cases they are 7 or 8%. The human body actually need abut 4% body fat to survive and work properly. So at a REAL 5% you wouldn't be able to perform athletically. When the picture above was taken I had to do 20 minutes workouts because I didn't have energy for more.
VIA SKIN FOLD CALIPERS a VERY GOOD contest shape bodybuilder would test at about 1mm at every body fat site. Which would actually come up to 1% or less but this actually impossible because the minimal functional body fat is about 4%. Maybe 3% is achievable for a very brief period of time but there wouldn't be much visual different between 3 and 4%. But at the lower level I'd say that most guys are A REAL 5-7% which would come up to about 2-4% on calipers and at lower level shows you often see guys who are 8 or 9%.
Again this shows how calipers aren't that effective. I often use myself as an example. At one point I was measured at 4.1% by a very reputable coach. But in my evaluation I was about 10lbs away from being close to contest shape. The following week at a seminar I was tested by over 15 good coaches and my body fat came up anywhere between 6 and 15%!!! And when I came back the guy who originally tested me, tested me at "unmeasurable" (less than 0%!!!).
Normally at 10% you have full on abs, veins across the shoulders and arms, good definition everywhere. At 7% you can only pinch some fat in areas like obliques, lower back and glutes. At 5% your skin is paper thin everywhere.
It depends on the individual. Some people have a naturally lower body fat than others. These people can have a lower body fat level and function well. For example someone who is naturally at 9% (you have people who eat junk and stay at 9%) then holding on to 7% year round is not a stress on their body.
But if you have someone who would normally be around 15-18% holding on to the same 7% will actually represent a big stress on the body.
It's not really the low body fat that is the problem it is what you have to do to keep the body fat low.
If someone could (God given genetics) stay at 6% bodyfat even when eating 4000 calories then he would be able to stay at 6% while adding muscle mass because he would be able to eat enough nutrients to fuel growth and keep his anabolic hormones high.
But if someone has to ingest 1800 calories/day and ingest less than 50g of carbs/day to maintain 7% he would not be able to add muscle because he would lack the calories needed to fuel growth and this body would produce more catabolic hormones than anabolic ones.
Again it's not a matter of the number itself but how fat it is from your natural setpoint and what you have to do nutrition and training-wise to maintain the low body fat.
But in general women are more at risk then men. When their body fat level drops too much their reproductive system is put on halt (they stop having their periods at one point) and are more at risk of developing ostheoporosis. This is known as the female athlete triad.
When a man goes too low relative to his own setpoint he will also suffer some reproductive issues. For them it mostly comes in the form of a loss of libido and even erectile disfunction.
There are also psychological issues that can occur.