I have a male friend, and our crowd always refer to him as the one true grown-up. It's hard to define, but I think it's because he's so reasonable and can see both sides. He never gets angry or flustered. He's nice, but not too nice and has a good sense of humour and he never jumps to conclusions. He is patient. He is on all sorts of agricultural think tanks, a leading light in the NFU and various other agricultural organisations, and was one of those to pioneer tracing the movement every step of the way from apples to bulls, grain to sheep long before foot and mouth really hit the headlines.
A true grownup is someone like him, who you would trust to run the country.