Depends on how one defines maturity.  It is very subjective. Each and every person's expectations of mature behavior is different I feel. In terms of being able to understand and realize the emotional state of others, perform actions without the intention of hurting anyone's feelings, having a mental makeup where we are objective when it comes to work, the ability to operate without wanting anyone's approval and being able to look at the bigger picture always. To add to it and make it simpler, i guess we shouldn't act upon jealousy, irrational fear, greed, envy, our ego.. anything that is impulsive, self destructive and can be brought under the ambit of compulsive behavior.  By this i do not mean that we should try and suppress such emotions, but rather make an effort to be self aware and understand the root of this behavior. 

A lot of it has to do with upbringing probably  and also the individual's willingness to evolve. Age is definitely not something that always determines a person's maturity. It is their willingness to learn and increase their self awareness.