Just been reading this thread. Took voluntary redundancy last May just after my 59th birthday. I haven't regretted it for one day. I'd never thought I would go as early as this as I'd been lucky enough to mostly enjoy what I did, but changes over the last two years had taken any pleasure out of the job. A new Chief Exec had ordained that everyone had to have a 'can do' attitude. Anyone actually using their practical experience to point out that an idea was dangerous, impractical, illegal, have unintended consequences etc. simply wasn't showing the right attitude.
She then insisted that my boss restructure and so he took my post out of the structure with my agreement. As I had 38 years pension and got redundancy it wasn't a hard decision.
When they realised how much it was going to cost to let me go they had a bit of a panic, and suggested that I could be allocated to a job one grade lower, which the VR rules allow, but knowing that HR always panic when challenged I said that if they tried that I'd put in a claim that my job should have been on a higher grade, which if successful would mean they couldn't make me stay in a job two grades lower and it would cost them even more in pension and redundancy . I never heard any more about it.
Returning to your original question. Not everyone enjoys retirement. About 15 years ago I recruited a chap who had retired a couple of years before. It was a part time project management role for which he was over qualified, but he wanted something to keep him busy and feel useful. He is still there!
Finally if you've got any sort of practical engineering skills then any heritage railway will be only to happy to have you, just be careful it doesn't take over your life.
Bob