Possible causes to explain why people physically age focus on the effects of stress hormones, telomeres, free radicals, bodily wear and tear, and genetic variability Muscle and bone mass decline throughout much of adulthood but more noticeably in later years

Declines in sensory functions such as vision and hearing may be problematic in later life Several changes to the brain have been linked to decreased cognitive functioning in older age

telomere — protective strand of DNA on the tip of each chromosome that becomes shorter each time a cell divides until the cell is no longer able to divide any more. cellular clock theory — theory that cells can divide to reproduce themselves only a limited number of times, and as they reach this number, the body is no longer able to replace old or damaged cells…. free radical theory — theory that free radicals, unstable oxygen molecules that are a by-product of food metabolization, damage tissues or cells, and this damage accumulates as people get older. presbyopia — age-related decline in vision due to stiffening of the lens that results in more difficulty seeing things that are close.