Cosmic

Welcome to the world of Liam, a twelve year old who looks like a grown-up, including the whiskers, and who has more smarts than he knows what to do with. 

It's his smarts that manage to get him in trouble from posing as a grownup in school to impersonating a fellow classmate's father in order to visit places children have no place being or doing things best left to adults-like purchasing a car--almost.

Looking older than he is has its ups and downs-like being mistaken for an adult and getting to do adult things and looking like an adult and being expected to act older than you are-even though you're a twelve year old boy.
Liam's masquerading as an adult really gets him into hot water when he poses as a chaperone for a group of kids on the first ever civilian spaceship flight.  Too bad Liam's not an adult-he might be able to figure a way back home when their spaceship malfunctions and they find themselves hundreds of thousands of miles away from Earth.
Cosmic is a delightful children's book with a unique main character, a quirky supporting cast and an outrageous and implausible story that author Frank Cottrell Boyce (I like that last name) manages to make perfect sense.  Look for Cosmic from Walden Pond Press.