17. I had finished school, and decided to work for a year or two before going to college. The job I got was in a different country to my parents, so I had little choice.
Shared small place with four others, which kept the rent down, walking distance to work. Lived very frugally because used all my spare cash for beer & concerts and saving up to travel... Work had subsidised canteen, which helped.
My mother cried when she found out I was even buying shoes from the thrift shop. Only saw them once that year, they flew back for my grandmothers' funeral, where I upset my father as I was wearing dark green. Couldn't afford to buy anything black, and so had just worn the darkest clothes I had.
Learnt how to cook lentils & rice.
A lot of it was just young pride - my parents would have helped if I asked, but I was determined to not take a penny. They very generously paid college expenses though.
My wife also left home at 17, that was to go to Canada to learn English & go to college.
I would guess that there will be a high % of "leave home early" on this board. For what is FIRE but a desire for independence?