I've grown up in a family where my brothers are all much older than I am, and people have always commented that I am mature for my age. I've already completed a university degree, have travelled extensively, lived on my own etc... Your case may still stand though. There is 8 years between us which is a decent amount of time. He may have viewed my way of dealing with arguments to be immature. I see it more as sensitivity than immaturity though. I'll take that into consideration.