Why I made this thread is that I am feeling quite alone while losing friends to the baby club. I have nothing in common with them any more. My life is same as it was five years ago but with few friends
The 30's and early 40's are the worst time for this as a childfree person. Friends do drop away. The children are very young need constant attention, parents are new to it and still learning to cope, money is tight due to childcare costs and babysitters a luxury, leaving the house is a military operation and nothing can be spontaneous. Conversations are boring child centred ones because they think their kids are fascinating and it's so overwhelming it's all they have head space for.
You do need to extend your social circle. Find new childfree/single friends, join groups with hobbies or interests you enjoy,and you will find there are many people out there. My childfree friends cover a wide age range and I Iove that. I'm in my fifties and youngest mate is just turned 40, oldest friend is in her 70's and is an energetic, intelligent, interesting person who has had an incredible life, still working and at top of her game.
As someone else said, it's not about "filling in time" it's about finding purpose in life and doing what you enjoy.
I travel a lot (well did...!) on group guided trips to places I'd never go to alone (met 2 now close friends that way too). House/pet sit for friends on holiday so I get to stay in differnt houses and visit nice parts of the country for a change of scenery. Even better now thanks to Covid I'm WFH full time and can be anywhere working. I can do things spontaneously, for example nice hot weather coming up ? Book some short notice annual leave and book a B&B for a few days. Or just take a day off and head for a park, beach or countryside walk. I get time to read, watch interesting TV, go running. Life has it's lonely moments and existential crises do happen but children would not have prevented those, in fact often just mask or delay confronting them.
Also many of my friends came back into my life as they emerged from the baby years. Many found that the friends they had made through baby groups they had nothing in common with other than the kids and that got boring. If you value the relationship just keep up the contact as best you can, a quick coffee in the playground cafe et (but NEVER say yes to softplay centres, you have been warned....). They will soon start to see you as a childfree respite centre with wine! If not, well it wasn't the strongest friendship in the world after all.