Touched a nerve with me OP. I’m 64 and haven’t had a full time job since I was 58. At that age I assumed I’d be a pensioner at 60, but then discovered that I’d fallen neatly into the pension age gap which means I don’t get a state pension till I’m 65 and 3 months .... so less than a year to go now. 
At that age I felt too old to start a new career, and it was too little time to top up my pension, so I was left a bit high and dry. No kids, so no grandkids to babysit, I have been grateful for a couple of highly time consuming hobbies, one of which I’ve turned into a profitable small business.
I’ve also fallen into volunteering for the National Trust. They definitely fall into the category of lots of take and very little give, but they have provided an outlet for my time and creative talents, a great new group of friends and acquaintances and a structure of sorts to my otherwise rather random life.
So, am I a useful idiot? I don’t think so. Useful yes, idiot no. My hobby business and NT work are both absorbing and creative in their own way. I feel respected for my abilities and contribution in both and adequately rewarded either financially, or in terms of the feeling of having made a useful contribution. So, I’m busy and interested in what I’m doing.
I was recently asked by the NT if I’d be interested in a volunteer role doing pretty much what I did for a (highly lucrative) living when I was working full time - marketing and PR. They tried to “sell” me the role on the basis of “using my skills and experience for a good cause” etc etc ..... I politely declined. Doing my old job for free did feel like useful idiocy.
I think it’s all about finding a new way forward. Applying oneself, being useful and interested/interesting, applying skills and experience, but doing it for ones own self fulfilment and not getting back into the daily grind - whether that work or looking after the grandkids. It is possible.