In 2005, I started what eventually became my current job, and I had two co-workers who were both lifelong bachelors. Rick and Gene were both in their 40's, as was I at the time. Gene had been with Nancy for 17 years, they lived together and were common-law spouses, but were not married and had no intention of ever getting married. Gene retired a few years ago and rode off into the sunset with Nancy. Rick and Julie had been together for a while, and three years later they were married. Rick retired a while back, too, with Julie. Their situations (the two couples, that is) were basically exactly the same. They'd been together for years, lived together for years, will probably die together. The only difference is that Rick and Julie finally decided "what the hell" and got married at 50something, and Gene and Nancy didn't.
Getting old sucks anyway, but I have to imagine that getting old alone has to suck more than getting old but having someone with you on the journey who can help you and who you can help. I come on DTF and bitch sometimes about my wife or women or whatever, but it's all just philosophical, just discussion.
And does it really matter if you're married or just "with someone"? No, not if it doesn't matter to you or her (or him). But I'd still rather be with someone than alone as I get older.