At what age do you think guys/girls REALLY mature?

I assume that you are referring to Mental Maturity and not the physical since it's not clear as you stress on "Really". Mental maturity basically depends on exposure in life. The current education system prevents the young from worldly exposure as they are harnessed by parents & education. So most of them never mature as they take everything for granted and think that's the way it is. Most display an attitude problem and possess an intolerant disposition. In the past 35 was the age for maturity but now the world needs an examination to issue a Maturity certificate, like driving license.