The one thing that “About a boy” really displays is that people’s level of maturity is not necessarily interdependent with their age. The book is about a child that acts like an adult and an adult that acts like a child. I think the title could even imply that the two “boys” that the book is about really have the same age, from the inside. Which is, when you think about it, not at all a rare thing to come by. The problem for the two main characters in the book is that this other level of maturity than is suited for their age makes them stand out a lot. They don’t blend in with other people their age because their state of mind is so different. Of course this effect is enlarged by he fact that the child Marcus also has a weird, unstable mother and the fact that Will has not got a job or a wife or children or anything average like what you would call normal for his age. But then again, you can question whether those facts are causes or consequences.
So what seems to be the deal here? Extreme situations force children to take on a more responsible role than that is suited for their age, and therefore it can happen that some children who for example are 17 years old are more mature than some adults that are over 30 years old. Also you have adults that never have seemed to get over their childhoods.
In other words, age is a relative phenomenon. Your physique alone can already totally differ from your age in years, but another thing that indicates it is the concept “body age” or, “real age”. Your body age is the age you have physically, meaning the state your body is in and whether that state suits your age in years. Doctors can calculate this for you with numerous tests. For example someone who is fifty years old but smokes and drinks a lot can have “body age” of sixty, because of the damage this lifestyle has done. If that same person would exercise a lot and eat extremely healthy its “body age” could be forty-five.
So how old are we, really? Nobody would mistake an old man for a teenager, but a factor that might just be as big as the physical ones is the mental state. How we act, talk, think and react in our daily lives says how mature we are. Use of vocabulary and body language, but also the way we solve problems and react to big events can easily reveal “the real you”. And that real you can be older or younger than your age tells us.