Why You Don't Seem Mature to Others and What to Do

Sometimes, we change the subject matter of the conversation unconsciously. What happens is that the person feels that we aren't paying much attention and that our mind/ thoughts are not stable. Therefore, it's best to stay within the topic.

Same as the other ideas, displaying a lack of self-control is another factor. Complaining about something because it didn't go in our way, feeling empty at hard times, aggressive behaviors are all features of low self-control and also low emotional intelligence. 

In this case, controlling emotions is necessary, or less outward emotional expressions can be assisting.

Interestingly, our getup also matters. In fact, in experiments, younger people tend to choose light colors and older choose darker. Flashy clothes also indicate childness. Even our hairstyle makes us look irresponsible. It's human psychology to judge someone based on their getup. 

So in this regard, one can wear something a little bit formal (not too much formal or too casual; a little above) and at least get a haircut that doesn't block their forehead like an emo kid.