Why is Jem so "inconsistent and moody"?

Jem is "inconsistent and moody" because he was growing up. He'd reached the point where childhood needed to be left behind and young adulthood embraced.... a very confusing and hormonal time of life for a boy.

Jem was twelve. He was difficult to live with, inconsistent, moody. His appetite was appalling, and he told me so many times to stop pestering him I consulted Atticus: “Reckon he’s got a tapeworm?” Atticus said no, Jem was growing. I must be patient with him and disturb him as little as possible.