I guess an immature person is someone who behaves inappropriately. For example, if there's a serious conversation, they'll laugh.
I do that sometimes. Because at times, someone else's serious conversation doesn't seem too serious to me and trust me, it's not my fault. Atleast I don't think it is. In fact, I sometimes wana tell them to freakin open their eyes and look at the world they are living in and not be so selfishly self absorbed.
Maturity and immaturity are very relative terms. In my experience, I found the one cracking the most jokes and laughing hysterically at nothing to be truly understanding.
I think it lies in how well a person handles conflicts-any type of conflict. Are they capable of pulling through or do they give up? Do they throw a big temper tamtrum or do they remian calm? do they get irritated by small things or ...so on and so forth