We generally refer to a blond youngster as a 'towhead'. Where does this term come from?

Tow means 'flax or hemp fiber', and so tow headed is literally 'flaxen haired'. This meaning of tow comes from Middle Low German touw (which means 'flax, hemp fiber').

http://grammarist.com/words/towhead/

A towhead is one with fair hair, typically white-blond. Blondness in youngsters is rampant while scarce in adults, as hair tends to darken with age. Know that this term is not restricted to a child. This relates to "tow," coarse broken flax or hemp fiber prepared for spinning. The word compares the colors of flax and fair hair. "Flaxen" and "towheaded" are totally synonymous.

https://en.wikipedia.org/wiki/Blonde