Small point, but have always understand geriatrician is the physician that specializes in medical care of the elderly and diseases/disorders associated with aging.

Gerontology can include the above but more the sociological study of aging. How society treats the elderly, the support systems, law, relationships with elderly, etc.

http://www.nlm.nih.gov/medlineplus/mplusdictionary.html

Gerontology:
the comprehensive study of aging and the problems of the aged

Geriatrician:
One who specializes in geriatrics.:
1 ge•ri•at•rics plural but singular in construction : a branch of medicine that deals with the problems and diseases of old age and aging people
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