My parents are in their 60s and 70s and have many friends who never married or had children.
As they've aged and started to have more health issues, their friends have stepped up to the plate. Even some of those who did have children have been supported primarily by their friends.
My dad has driven his friends to and from chemo treatments. He visits them at home and brings them food. He's helped them arrange for Meals on Wheels or hospice. Many of his friends do the same for each other. They've known each other for 50 years and they all look out for each other, with the healthiest ones looking after the sickest.
My dad's childhood friend Jay passed a few years back. He never married and had no children. My dad and his friends cared for Jay for years as he deteriorated, and he died at age 70 with his mother still alive at 90. My dad then took over looking after Jay's mother, since Jay was an only child and they'd been friends since high school.