So what does maturity and immaturity look like in adults? It's important to note that although it's common to think that once we reach adulthood, we're mature, the reality is far from it. The reality is that we all develop in maturity throughout life, but never fully reach it. Reaching adulthood only guarantees physical maturity, but certainly not emotional maturity.
So, you want to know what maturity looks like. Here goes:
Maturity looks like experiencing strong emotions but not letting them hijack your behaviour. An immature response might be big emotional outbursts when upset (e.g. screaming at others, or becoming aggressive when things don't go how we want them to)
A mature attitude would be avoiding blaming others when things don't go our way, but instead considering how we ourselves have contributed to the situation. Needless to say, immaturity is marked by looking for fault in others when something goes wrong, and making sure others know they are to blame
Staying in relationship with people who disagree with you or have differing views is a mark of maturity. It is easy to cast off friends or relatives who don't see eye to eye with us because it's uncomfortable, but by doing this, we deny ourselves an opportunity for mature growth (not to mention the relationship itself)
A mark of maturity is taking responsibility for solving your own problems. Demanding that others find solutions to our problems, and making it their problem, is probably a sign that we've still got a bit of growing up to do
Holding onto your values under pressure or in the face of conflict is a mark of maturity.
Recognising that your needs are not particularly concerning for most people, and responding to this with grace, rather than demanding, 'What about me?!' is also a hallmark of maturity.