It is generally accepted that men should have less than 18% of their bodyweight as fat and women 23% or less. A certain amount of body fat is essential for good health. There is plenty of evidence to suggest that carrying less than 5% body fat compromises your immune system, making you prone to illness and infections.
Athletes in training, especially at the elite level, will have significantly less body fat: around 8-10% for men and 10-12% for women. High levels of fat in relative terms are a serious disadvantage to most athletes, especially in disciplines where “making weight” for a specific weight class is a priority.
Carrying more fat than the average person is not particularly hazardous to health until you accumulate 35% (men) or 40% (women) of your total bodyweight as fat. Such levels constitute obesity and have a detrimental effect on health.