I like Dr. Daniel Amen's 18/40/60 Rule: When you're 18, you worry about what everybody is thinking of you; when you're 40, you don't give a darn what anybody thinks of you; when you're 60, you realize nobody's been thinking about you at all.
Jack Canfield, Kent Healy (2010). “The Success Principles for Teens: How to Get From Where You Are to Where You Want to Be”, p.29, Health Communications, Inc.
