For my entire adult life I've had pale circles under my eyes. It is most noticeable when I'm flushed or tanned. What could cause this?

Variable skin color. Your description sounds like a variation of normal skin coloration. If this bothers you, try makeup concealer or laser skin treatments.
Lack of pigment. You may have just been born with a relative lack of skin pigment in the skin of that area. The skin under the eyes also tends to be thinner on that area of the face. If you look closely at the skin of people's face, you'll see there is a wide variation of what appears. There is no one "normal " appearance. We all don't look like the photoshopped models in advertisements.