There is no
need to act mature. Its sad that when you meet someone that can
speak well, or have a higher standard of English, you seem to
believe that it must be an act.
Not too sure why, maybe its the fact that you never had any friends
with a command of English better than that of a primary school
student.
Trust me on one thing, not everything must be an act. Some people
are actually mature. Wake up and smell the coffee.