The quote emphasizes that when someone is young, they have the power to shape their future character through their choices and actions. Choosing to be kind and decent can lead to a positive and compassionate personality as they grow older. The notion that one might remain mean if they are mean as a child is not something the speaker considers, highlighting the potential for change and growth.
It suggests that personal development is flexible during childhood, and the way a person chooses to behave influences who they become. Kindness and decency are presented as qualities that can be cultivated early on, shaping a person's future rather than remaining fixed traits. The message is one of hope and the importance of conscious choices in growth.