I mean, the MC also acts kind of stupid. Why is he taking the way of gaining mangekyou from the show as gospel. Also I really don't see it working if he doesn't truly care for his family or sister.
if chakra is the mix of spiritual and physical then it makes sense his chakra reserves will be low until he gets his first growthspurt. And then again during his teenage years it should also grow the most since then is when msucles grow the best.