This has been where my thoughts seem to converge in the last few years too. I'm days away from 40 now, a parent for 17 years and a week or so.
The things that distinguish the adults from the children in my life tends to be age less and less, and responsibility and accountability more.
Your ability to be of service to people in your family, your circle of friends, and your community is such a great measure of how you've become an adult. It isn't a perfect measure, but the best proxy I've found. It's very difficult to spoof it.
People who have aged but failed to mature tend to struggle on all or several of these metrics. Their attention, actions, and overall lives are very inwardly oriented in many ways.
It isn't to say they're bad people. My rough framework is that as social animals, we need to figure things out before we can fully integrate into our social systems as a fully functioning member. A big step in this process is figuring ourselves out. That's why kids are doing legitimate work when they play, make mistakes, struggle, and so on. They're doing those difficult steps of self discovery. Then, we need to figure out how we fit into the social layers and meshes around us. It's all very complex. It's understandable that we never fully figure it out or optimize, and that some people get hung up on early steps without the right help to be guided through. If the foundations are poor, you're going to struggle.
In effect being an adult is just being a well adjusted, integrated member of a community who functions as a generative, supportive, all around positive contributor.
That's an over simplification of course. It's a proxy I use to help guide myself, really. What can I do right now that would land me in that rough category? It's helpful to not have to overthink it.
At 40 I'd say I've begun becoming an adult but have a lot of work left to do. I think the efforts need to be ongoing because we never stop teaching the young ones, too. Complacency in some contexts can be totally fine, but in a social context I think it can be quite corrosive. We always need to care and strive for something better for each other. It's what we're here for.
Also, nice work, genuinely. Parenting at 23 was a shock to me. People thought I was handling it gracefully but it was totally ad-hoc and incredibly difficult at times. And that's with a disproportionately advantageous tech career supporting us. I can't imagine what would have happened at 15. That's admirable. I didn't even make it through school without a kid. I'm regularly amazed by what people can accomplish, including this.