How does one differentiate between 'old age' and dementia?

If Mom can't remember how to use a computer after teaching it, she has Dementia. My boss, RN, discribed Dementia as...knowing its a stove but forgetting how to use it. Alzheimers...you forget its a stove.

I agree, Mom needs a full physical with labs.

Of course you are already, as an adult, aware of the problems of the elderly with eyesight, mobility, strength and etc. So we can just refer you to sitting about "watching the world move by" as far as the problems of normal aging process.

Dementia is defined by mental deficits that make it dangerous for elders to attempt to function safely on their own.

Remember, the internet is your friend, and if you type into your favorite search engine, Google, Bing or any other the words "What is dementia" or "What is normal aging process" you will get a whole WORLD of wonderful help.
Also talk to your local librarian.

Older people may be slower both physically and mentally and that might cause difficulty getting stuff done in a timely manner or learning new things, but aging alone shouldn't cause loss of previous knowledge or the inability to do and understand things we have previously mastered.

The following is a link to an article discussing the differences between normal aging and dementia. The only way to know for sure is to be tested.

https://www.nia.nih.gov/health/memory-loss-and-forgetfulness/memory-problems-forgetfulness-and-aging

Would you mind listing some things that she does?

Dementia is more about loss of executive functions along with memory impairment. A lot of people don’t realize that about executive functioning.