My 8yo - whenever she wants to - she showers herself, and likes to look "nice" so looks after her hair.
6yo - whenever I remind him to, otherwise he would never wash it! He also showers himself. Prob twice a week.
My 10mth old - whenever I remember! Maybe a couple times a week, or whenever he has food in it!
I only wash my hair twice a week in winter..my hair is quite thick and takes ages to dry, so its a hassle, and I dont have time to use a dryer in the morning! I don't think I smell