It's really harder to make friends in adulthood. Especially when you already have a family, you are building a career and you do not have much free time.
From my experience, I can tell you how I started a friendship as an adult.
It wasn't planned, I wasn't looking for friends. I just did repairs at home and I was recommended a team of workers. The repairs took a long time, about 4 months, and I was almost always in the house, so I had to communicate with the staff. And one of them turned out to be a very interesting person, we had many common interests. In 2 months we started to invite each other to visit, to ask for help. The renovation finished, but our friendship remains.