I've actually spent a large amount of my life alone.
If i'm honest I would have to admit to feeling lonely quite a few times.

Despite my life being very busy and involving myself in many teams and much world travel I still did feel lonely and isolated from time-to-time. Especially when I was required to base myself in foreign parts for say 6-9 months at a time. Have to admit under those conditions I didn't always take sufficient good care of myself. Not that I ever got really ill, just sometimes I didn't eat well or I missed out a meal, sometimes slept too much sometimes not enough. Sometimes drank too much beer and sometimes did pointless things to save the extra money.

It's not just the elderly that can suffer loneliness and other associated impacts.

I have a few neighbours that I regularly check on and spend a little time to be sure they're OK, but by and large even in our village there are many people who don't know their neighbours.

Interesting subject, and the way our 21st century life is headed with TV/Radio/Internet/Phones etc etc the more people can live distanced lives.