Freemond Dyson, an 88 year old physicist, mathematician, and generally smart person:
Advice to people at the beginning of their careers: do not imagine that you have to know everything before you can do anything. My own best work was done when I was most ignorant. Grab every opportunity to take responsibility and do things for which you are unqualified.
Advice to people at the middle of their careers: do not be afraid to switch careers and try something new. As my friend the physicist Leo Szilard said (number nine in his list of ten commandments): “Do your work for six years; but in the seventh, go into solitude or among strangers, so that the memory of your friends does not hinder you from being what you have become.”