Not always true. Some 18 year olds have a more mature outlook than some 30 year olds! It completely depends on the individual. Also I don't think two people necessarily have to have exactly the same level of maturity in order for them to be friends.
True and I did read some article where this woman talking about the Flack situation and had met Harry and said he was years older than his years and when you met him you forget how young he is. I would imagine he has had to mature pretty quickly in the last two years with the job he has.