I've got a few older friends who have always been basically single and don't have any children.
One thing they have in common is good financial planning. One has a property she rents out, and at 50, after two careers, took on a business franchise to prepare for retirement - something with employees, something she could manage, rather than hard graft. Another has a research job she loves and has progressed well with no other ties to consider. Her pension pot is great and she owns her property outright. (Obviously that's a quirk of timing and the property market is not what it was.)
All of them have a core of friends who share interests but are quite compartmentalised by interests - rather than a group of friends who all get together all the time, they have friends for theatre, friends for tennis etc. They work at maintaining those friendships and are proactive at making arrangements. FAR more so that the people who have children and partners and cancel at the last minute - it's really noticeable.
I have another friend who's younger and struggling a bit with this. She's not working at being financially solvent - I appreciate it's harder when rents are ridiculous - but she won't, for example, work at a second job for a while to get over the hump. She's accepted that things will always be hard financially and I don't agree with her approach to it, especially since there is no magic wand here, she's got to manage the rest of her life. There's a lot of 'other people have it made' and not a lot of 'hmm, I wonder how they did it and what I can do?'
I've rambled on a bit but from what I can see, getting yourself financially self-sufficient with a plan and a willingness to explore opportunities seems like a good way to go if you don't have a partner.