To me, you are NOT an adult if you hit your mid 20s, or even your early 30s. Yes, you are legally considered an adult and you set out to start a career, start a family or do whatever the hell you want in your life. But you are basically getting the hang of things, learning the truth of life and finally figuring out who you really are. In order to be considered an adult you have to be in your 50s or even 60s since by then you have experienced many things.
No matter how "smart" or "wise" a 20 or 30 year old might seem, they are still kids trying to figure out life. Hell, you could have experienced all of life's harsh realities at an early age and that STILL doesn't make you a mature adult. That's something that takes years to form.