I am not a 20 something, but I still find this topic interesting, so I'm chiming in with my thoughts. I hope that is ok.
I guess it depends on how you define "adult". None of us ever reach a magical moment when we automatically know exactly what to do in every situation we face. That's an impossible dream.
But, with a bit of experience under your belt, and by building a good support system over the years you can start to figure out the pros and cons of acting certain ways in certain situations. You can learn tools for managing upsets, frustrations, problems, etc. and those tools can help you deal more successfully with those issues when they arise.
Part of growing up (just in my opinion) is realizing that things will never be "ideal" or "perfect" or "just how you imagined them"... and being ok with that. Life can still be wonderful and enjoyed, and we don't have to act like toddlers throwing fits when things don't go just how we wanted them to.
I don't know if this helps, but it's just my 2 cents on the topic! I don't think being an adult has anything to do with being married, having kids, having a certain job or living in a certain place. It's all mental and emotional. I remember being younger and really looking forward to the day when I would be "a grown up". Then I found out that you don't automatically grow up. It takes work to get to that place mentally and emotionally, but I think the work is worth it.