I've heard older people who are single say, "I'm too set in my ways to move in with somebody now," and I think there may be something to that. After living alone for a long time you adapt to a certain way of doing things, and it's hard to imagine getting used to sharing your space with somebody else.
Living alone is not necessarily the same as being alone. You can have friends, a social life, and a dating life without having a second person sharing your living space. Some people are just happier that way.
I'm 50 and I find I'm generally happier living alone. Maybe it's just a function of the bad relationships I've been in, but at this point I can't really see myself moving in with another person. And I'm happier this way, probably 80 or 90 percent of the time anyway. I like having the place to myself and I like being my own boss.
The downside is that there's times when I could use a little conversation, or just a second pair of hands to help with the grocery shopping and housecleaning. If you get sick, living alone just sucks. And I think, what if I got hurt? What if I fell down in the shower and hit my head or something? What if, what if? That's the part of living alone that scares me.