The answer to this to some extent depends on who the person is, and what the behaviour is, and how much their behaviour affects you and in what ways. Obviously this is a very broad question but there is an important point behind it, in terms of an individual's willingness to let other people be themselves and that other people can be mature or not depending on their age and life position.
The issue of maturity or the lack of maturity can often be seen as a moral issue, and immaturity can sometimes be used to denigrate people, especially if they are adults, where the term immaturity is often used to imply that they are acting in a childlike manner.
Often there is nothing wrong in acting in a childlike manner, depending on the situation and the context within which such behaviour is expressed. People often get upset not so much by other people's maturity or lack of maturity but by feeling that the behaviour is in some way reflective towards themselves.
Maturity is often seen as a process that children and then adolescents go through before they reach adulthood, and as such mature. Maturity in a literal sense is normally taken as meaning stages of development by the individual has to take to reach their potential. In general terms, maturity is normally thought of as a fixed point.
In human beings maturity has no real fixed point. People's understanding of maturity can differ widely, and is very much aged dependent.
This is particularly well expressed in literature such as that produced by Al-Anon, which lays a heavy focus on an individual taking back control of their own life in the context of a co-dependent relationship in active alcoholism, and learning to take responsibility for their own actions and beliefs, and not take responsibility for other people's behaviour or beliefs.
Learning to differentiate one's own life from other people's, and separating out what is your life from other peoples at an emotional level is often at the heart of what many people would define as maturity. Obviously mature behaviour follows on as does immature behaviour from whether the individual considers themselves mature or not as a person.
In families where active alcoholism has been an important dynamic, is often said that the alcoholic and people around them are in some way emotionally stunted by way of development to the point where they remain immature and unable to express themselves emotionally relative to that given age.
Whilst this may be true for many people who are not alcoholics as well, the ability to become more mature emotionally is open to all who genuinely own where they are at as a person, and become willing to take responsibility for their lives and their own actions.
Peter Main is a freelance journalist and copywriter who writes extensively about all areas of self growth and self development. He has a particular focus on self help issues for people who are in recovery from or who have been affected by alcoholism and other addictions. Some people begin their journey of recovery and healing in a rehab, others in a twelve step fellowship such as Alcoholics Anonymous, some in rehabs in other states to where they live, where it is especially important to check the rehab is properly accredited, others in a religious or spiritual setting.