Hello, Both obesity and the eating disorder anorexia can cause a late start of menstruation. Lifestyle factors, such as excessive exercise, and mental, emotional, and physical stress can cause late periods. Physical and lifestyle factors are common reasons for missed periods. These can include: obesity,having low body fat,exercising too much,having serious anxiety or stress,losing weight rapidly. Harmonal imbalance in pcos can also cause amenorrhea. Consult gynaecologist.