I Forgot the Name of That ___! discussion

It was a book I read in middle school, 02-03 ish, and it was older. Actually it was a series of 3 books. It was about aliens that had already taken over the earth, and once a person turned 16 they had to get a cap or something put on their head. Main character is a boy who is close to turning 16 but when he sees what it does to his brother he runs away finding a rebellion and joining it. The aliens are really tall and have 3 legs.
Halp?

this is the white mountains by john christopher

That's it! Thank you!!!