Ok this is all doing to sound very robotic i think, but I've been pondering my own friendships recently. I'm 36, married and have a preschool aged dc just to put in context.
I tend to make friends through work, hobbies and kids play dates with parents i click with. Friendships for me at the moment are based on proximity and frequency of seeing them, not necessarily history or similarity. As a result I have a really varied network now of people who have very different interests and views but i guess we enjoy each other's company on some level.
Proper friendships have then developed where we've both shared feelings/issues with each other about things we're going through, it seems to be an act of bonding where even just the action of talking and listening can feel like support. But obviously not treating new friends like counsellors!
I think my friends and i talk about shared experiences (parenting, work issues, partners, stuff we're doing), learning about each other (all the stuff we don't have in common - their job, where they've been on holiday etc.) and helping each other (ideas, problem solving, recommendations and sometimes actually helping with things like child care if needed).
I tend to be open and friendly with new people, try to have a sense of humour (not that it's clear from this!) and keep things breezy. Also maintaining contact and being reliable with meet ups is pretty important.
That's what works for me.