I also don't think it's an age thing. People who are say 65+ who moan and gripe, were moany feckers to begin with IME. I know some lovely positive people over 65, and some whiny ones.
My mother was the same, moan whinge, whine, about everything, from the neighbours, to my brother rarely contacting her, to her health, to the government, to tv programmes, ANYthing. She would moan about anything. And OMG she didn't half moan about all her health ailments! 
I find the ones who moan and bitch and whinge and gripe, and slate you if you don't contact them constantly, every day, often have nothing else in their life.
@retroginglass Could you try and get your DM interested in a hobby group? Maybe try and get her to buy a laptop and get her interested in the internet. She could even play games on her laptop. Or go on a penpal website to try and meet/connect with new people. Or buy her an old Playstation 2 and some puzzle/platform games. Anything to give her something to do.
My DD's boyfriend's nan is like your DM. She is 70, and has NOTHING in her life. No hobbies, no interests, no friends, no TV, no computer or anything. All she does is knit. One hobby that's it!
She complains profusely if one of her 2 adult children (in their 40s) or one of her 4 grandchildren (aged 18 to 23) don't ring her EVERY SINGLE DAY. And at least one of them must go see her 4 or 5 times a week. One of the adult children went a week without ringing her or going round one day in summer 2019 as she had flu. The woman ghosted her for a MONTH. She told the rest of them to tell HER she is in the doghouse, and banned from calling her or seeing her for a month.
The mother didn't contact her even though she knew she was ill. Baffling!
Her daughter - DD's boyfriend's aunt - said it was the most peaceful month of her life without her mother hounding her though. Also, she had to say sorry to her, before she was allowed back. Fuck that for a game of soldiers. I am secretly hoping DD doesn't get too serious with this lad. Don't want her marrying into a fucked up family dynamic like that! 