My hair gets dry and breaks if I wash it every day. Also, too much shampoo makes my scalp flake (and I use Head & Shoulders!). My hair always looks best on the second day, and it is still pretty good on the third day. I like to bathe every day, but I take care not to get my hair wet.
Every 2 to 4 days works for me, depending on the humidity outside. (Less washing in the winter, more in the summer.)
I judge when it needs washing based on how it feels. I have very thick, wavy hair.
I think it's different for everyone! I don't think it's changed as I've aged, either... I think I've just learned how to deal with it better. My mother (very fine, light hair) used to make me wash my hair every day.
Oh... and my daughter's hair is like mine! Straighter, but just as thick, if not thicker. She washes her hair every other day. My son's is... well, it's pretty much a fluffy Afro (a Franco-fro, actually). He'd never wash it, if I didn't make him. Three-four days it never looks any different. I aim to make him wash it every other day, but sometimes he manages to slip by me.