“What are the symptoms of being a hoarder?”

Not necessarily. Older people lose their family and friends over time. Their world contracts around them. Their now-expanding empty time becomes filled with memories of the past. Objects become treasured memories of past relations as a substitute. At some point, this can be so severe that she may become a hoarder. She does not want to lose anymore in this time of loneliness. She needs active relationships. Relatives much make much more time for her. Encourage assisted living arrangements where she can find company and new relationships. Get her involved and off this destructive (but all to common path). Some governments have day time services for the elderly where they can go & become involved. Best check with psychiatric care to see how bad this is and what services will be needed.
Best wishes. We want her life to end happily and feeling a part of family & friends, not in lonely despair!

Yes, they are. Read about obsessive compulsive disorder. Look for help from a mental health provider.

This link should provide you some insight into your Q
https://www.everydayhealth.com/anxiety-pictures/9-top-signs-of-hoarding.aspx

Yes to a point. As we get older, we at times hold on to things that reminds us of the past and better times. Let her do her thing but you can remind her about hoarding