Between 75 and 90 per cent of erectile dysfunction is caused by physical changes affecting blood flow and slower nervous response in the penis.
This decline can usually be traced back to metabolic and physiological changes related to:

  • Narrowing of fine blood vessels due to atherosclerosis (hardening of the arteries) and other cardiac damage
  • Lower testosterone affecting brain chemistry and nerve response

Life style factors like

  • Weight gain
  • Stress, anxiety or fatigue
  • Lack of exercise
  • Smoking
  • Side effects of common medications (eg beta blockers or anti-depressants)
  • Heavy use of alcohol or drugs

Younger men sometimes develop anxiety over their sexual performance. This is usually not erectile dysfunction but performance anxiety.
For more information on some of the causes of erectile dysfunction see Common Causes of Erectile Dysfunction