Some say that success is measured by happiness. Maybe not a perfect yardstick since some of the dumbest people are the happiest people. I guess that kinda' shoots down the happiness yardstick.

A potentially useful yardstick is the financial well being to live safe and comfortably, not lacking material needs. Maybe that along with the intellectual capacity to know what you need and what you want and the skills and ability to acquire both. And also important is being smart enough so that other smart people don't think you're dumber than a box of rocks.