Biological Ageing

External Ageing refers to the superficial symptoms of  growing old. It includes but is not limited to:

• Wrinkling of the skin, which is exacerbated by sunlight and smoking. 
• Baldness and/or the hair becoming grey.
• A change in posture - as the discs in the spine shrink, older adults become shorter.

Internal ageing refers to the invisible degenerative changes that occur within the body.

• The brain shrinks in size (Peters, 2006)
• Cardiovascular system - atherosclerosis occurs when arteries shrink & harden making it harder for blood to flow (Seely, 1989)
• The respiratory system experiences a reduction in efficiency (Krumpe, Knudson, Parsons, Reiser, 1985)
• Urinary system - 'urinary storage' issues and incontinence, but this might be 'age-related' - [i.e. also related to poor health in old age] rather than 'age-dependent' [caused by ageing] (McGrother et al., 2006). 
• The immune system becomes less responsive and efficient (Dorshkind, Montecino-Rodriguez, & Signer, 2009)