When I had my first child, I was 16.
My friends were DEFINITELY "co-parents" though I had no idea that term existed back then.
In point of fact, many of those highschool friends, who I don't even keep in close contact with anymore, still remain in DAILY contact with her (she's 19).
They've retained parental-like relationships with her over the years even as our own lives have moved apart!
Some of them (who still live in the state) were at her graduation, without having received an invite (she didn't send formal invites), because they knew when it was and it meant something to them to see this kid who they've loved and cared for all of her life "succeed".
My ex-girlfriend (who I only mention once in a blue moon on here) remains in contact with my daughter. In fact, the only time I've heard from her in YEARS-was simply stating that she wanted to be in touch with my daughter but didn't want to remain in a contact with me and wasn't sure if that was acceptable. It is. She's a WONDERFUL woman with a great understanding of how to love. I would never want to keep her out of my daughters life, if my being a part of her life is too painful-so be it.
I think it's hard for most people to comprehend the idea that their children are individual people who can (and will) build relationships of their own, if given the opportunity.
But-for those of us who can, it's such a relief to know that if somethign happened to me (God forbid), my daughter would not only have our family, but her "chosen family" to hold her up, help her out and stand by her side through the difficulties.
It's such a blessing that damn near anywhere that kid goes in this whole country, there is SOMEONE who loves her near enough to help her out in a pinch. Even though it's not me.
And-on the other side of it-
all of those people say that it's been a blessing for them, that they've been granted a gift beyond comprehension in having the chance to have the love of a child in their lives that they otherwise wouldn't have had.
Some of them have their own children, some don't. But they've all found it to be beneficial to them to have this loving relationship with another child.