You are never too old to set another goal or to dream a new dream.
If you are holding on then you are trying to keep your hold on something. If you fall off of a high cliff and you are holding on then you are hanging on the edge. Letting go means plummeting to certain death. Sometimes the phrase holding on refers to a relationship that may be in trouble, and this is a relationship that you do not want to end so you hold on as long as you can.