The body changes in every phase of life. It reacts to activity, nutrition, environment, emotions, and so many more actions. The body is an effective action/reaction mechanism, it is a fast response unit capable of adjusting too nearly every situation. The human mind is made for adaption on the highest possible level.

Age has a strong changing effect on body and soul. Here is a physical explaination

But there is partly lose control by most individuals after 50. The body of the city-dweller increases in fat% even with a proper lifestyle. Nutrition and environment are important and intrusive factors. Here is an oversight of the different organs and glands with their change during an increase of age.

Organ or SystemPhysiologic Change
Body composition↓ Lean body mass ↓ Muscle mass ↓ Creatinine production ↓ Skeletal mass ↓ Total body liquid
General cells↑ DNA damage and ↓DNA repair capacity ↓ Oxidative capacity Accelerated cell senescence ↑ Fibrosis Lipofuscin accumulation
Central Nervous System↓ Number of dopamine receptors ↑ Alpha-adrenergic responses ↑ Muscarinic parasympathetic responses
EarsLoss of high-frequency hearing  ↑Hair growth
Endocrine system↑ Insulin resistance and glucose intolerance
Menopause, ↓ estrogen, and progesterone secretion ↓ Testosterone secretion ↓ Growth hormone secretion ↑ Bone mineral loss ↑ Secretion of ADH (arginine vasopressin/antidiuretic hormone) in response to osmolar stimuli
Eyes↓ Lens flexibility ↑ Time for pupillary reflexes (constriction, dilation) ↑ Incidence of cataracts
Digestive tract↓ Splanchnic blood flow ↑ Transit time
Heart↓ Intrinsic heart rate and maximal heart rate Blunted baroreflex (less increase in heart rate in response to a decrease in BP) ↓ Diastolic relaxation ↑ Atrioventricular conduction time ↑ Atrial and ventricular ectopy
Immune system↓ T-cell function ↓ B-cell function
JointsDegeneration of cartilaginous tissues Fibrosis ↑ Glycosylation and cross-linking of collagen Loss of tissue elasticity
Kidneys↓ Renal blood flow ↓ Renal mass ↓ Glomerular filtration ↓ Renal tubular secretion and reabsorption ↓ Ability to excrete a free-water load
Liver↓ Hepatic mass ↓ Hepatic blood flow ↓ Activity of CYP 450 enzyme system
Nose↓ Smell ↑Hair growth ↑Saliva increase
Peripheral nervous system↓ Baroreflex responses ↓Beta-adrenergic responsiveness and number of receptors ↓ Signal transduction ↓ Muscarinic parasympathetic responses Preserved alpha-adrenergic responses
Pulmonary system / Respiratory system↓ Vital capacity ↓ Lung elasticity (compliance) ↑ Residual volume ↓FEV1 ↑ V/Q mismatch
Vasculature↓ Endothelin-dependent vasodilation ↑ Peripheral resistance
↓=decreased; ↑= increased; 

Adapted from the Institute of Medicine: Pharmacokinetics and Drug Interactions in the Elderly Workshop. Washington DC, National Academy Press, 1997, pp. 8–9.