I think it's a very nebulous thing to try and define. I have friends who worked full time and lived alone from 16 - their choice, not evil parents or anything, they just got onto really good training schemes at BAE, moved into rented houses, bought a few years later - and were paying their own bills, for driving lessons, generally living as 'adults'.
I moved out at 17 and had no financial or practical help from then on. Short on rent at 19? Had to work 2 jobs around uni to manage. If something went wrong - my house flooded, I got ill and ended up in hospital, etc - I dealt with it myself or with the help of my friends. I didn't see it as my parent's responsibility to help me (much as my mum I'm sure would have loved to).
My sister lives at home, has never paid rent in her life, if she needs a new car my dad finds it for her and she hands over the money, she's never taken her own car for an MOT. Doesn't cook or do her own washing. When she was stuck abroad during flights being grounded, she called my mum and my mum sorted out another hotel for her, rearranged her tickets home. She wouldn't be able to tell you what council tax costs or how to sort it if the fuse box goes and you're sat in the dark. She's 28 - while she has a job, and a car, and goes abroad by herself etc etc, she doesn't feel like a 'real' adult to me, and a lot of her friends are the same. (And in a way, fair play to them - would love someone to come and do my washing and cooking! Just...not my mum cos she'd drive me mad haha).
I think it's generally when you're financially self-sufficient, able to mostly handle unexpected life things thrown at you, and don't rely on other people for basic life admin (bills sorting, getting to/from places, arranging housing, etc). That's what I'd class as actually 'adult', regardless of the legal definition.