Maturity to my opinion is how you take situations like you have a big fight an you act out an yell an etc is immature, cause as you said, a 26 year old you should know the mature an responsible way is to calmly answer even how frustrating the situation is, its action an how you handle it. Maturity is also a state of mind of common sense, an how well you sort out thoughts an carefully control them to not make them into immature actions of displeasing or hurtful things. Immatureness is undeniable a lot people are immature in own ways it's just how you define the majority or mature an whats not mature.