The aging process has you firmly in its grasp if you never get the urge to throw a snowball.
Most of our daily lives consist of habits. The way we speak, what we eat, see or do. Routines make us comfortable, but it deprives us of new opportunities and experiences. We should make it a habit of doing something new every day. Take risks, this will allow us to be more mentally alive. This quote is telling us that "if you never get the urge to throw a snowball" a fun and exciting activity, you are set firmly in the aging process.