When one becomes an adult, most often friends are made at work. That can be a bit trickier, because we often don't get to choose who we work with. Or through volunteer work, which is how I met a lot of my friends.
Since I moved around a lot growing up, I'm always envious of people who have friends from childhood all through life. I have a few with whom I've reconnected on Facebook or Instagram, but we don't truly know each other the way we would have if we'd spent years as friends in person.