U.S. Government sites:
The U.S. National Institute on Aging (NIA) has a collection of fact sheets and other publications available on a variety of health and fitness topics.
For information about Alzheimer's, try the National Institute on Aging's Alzheimer's Disease Education and Referral (ADEAR) Center.
Academic research:
AARP has a database of current literature on aging
National Resource Center on Nutrition, Physical Activity & Aging is a resource for professionals working in the field of long-term care, but if you want to get your information from the horse's mouth (the academic papers), try the above listed site. The Dietary Guidelines topic, for example, tells you about the modified food pyramid for the over 70s.
The American Federation for Aging Research (AFAR), a national, nonprofit organization that supports basic aging research, has launched a website to "provide easy access to authoritative and current scientific research on aging and age-related diseases and conditions".
http://www.afar.org/learn
Special interests:
For financial aspects of aging, try
https://www.aarp.org/aarp-foundation/
An article about Common Online Scams with tips that seniors can use to protect themselves from online scams
https://consumer-champion.org/resources/common-scams-targeting-seniors/
Seniors USA is an advocacy group, with a particular interest in the cost of prescription drugs and better health care. Their site, while emphasizing these subjects, offers articles on a broad range of topics assumed to be of interest to their constituency. http://www.senrs.com/
National Council on Seniors Drug & Alcohol Rehab is a non profit organization with a mission to educate and provide assistance to seniors struggling addiction as well as caregivers and family members with concerns. https://rehabnet.com/
For help with computers and the Internet, try