have most adolescents achieved their basic maturity which makes them behave like adults ?

your calender Age as nothing to do with your Maturity.

Maturity has to do with the experience that one gains in life
and also more so what one LEARNS out of that experience.
some aren't lucky enough to see enough life at a young age.
some others are lucky enough to see a lot but they are many times don't look close enough or don't think enough to gain that experience which takes them towards maturity.

i believe that mature kids grow on to become wise adults.

john Stuart mill said this once,
" LIFE IS ALL ABOUT EXPERIENCES AND I AM GLAD TO HAVE IT EARLY IN THE GAME "

another really good quote is a one by Carl Gustav Jung,

and i am sure he would agree that most human beings do not even experience that phase of life when they experience doing things because they haven't 'researched' enough, which means not had enough experiences, also not learnt enough or most of the times, kept themselves away from the hardships of life and always avoided moving away from the comfort zone.