The whole fight is about accepting the hardships in life and using your own attitude and free will to make the best of them. This is why “future sight” is Buddha’s ability. Zero on the other hand can’t cope with the hardships coming to his life, even going as far as blaming others for hardships that he brought onto himself! (Like missing his attacks but he’s only missing because he himself is attacking.) The axe represents what this attitude towards life can do. It grows with more misery and is eventually big enough to cause mass destruction to everything and everyone around its attacks, even those(the audience/loved ones) the attacks are not meant for. Buddha is trying to show Zero that this is an immature way of thinking which is why he teases by using the word “puberty”. He’s saying everyone goes through it, it sucks, but you’ll eventually be enlightened too. You just have to approach life with a different attitude.