At the age of 15 I was expected to be responsible for my actions, and rightly so, imo.
That wasn't what I asked but I do understand what you are saying, it was the same for most of us and for our children, we were expected to be responsible up to a certain point at different ages. For example, at 15 I was more responsible than my 12 year old sister. Parents and school teachers would give us more responsibilities.
However the responsibilities came with guidance, we were not expected to think like 25 year olds.
I never got into any trouble and was considered to be fairly sensible and mature but I know my thinking - my passions - had different emphases to when I became an adult. Not surprising, I was at school and hadn't yet even done my 'o' levels. My experience of life and my knowledge, which I considered to be pretty good at the time, was quite different to later on because I still had a lot to learn.
That's why we have juvenile courts and young offenders institutions, the law recognises youth. It wouldn't surprise me to know that your ideas were fully formed, or nearly, at 15 though. I'm just saying it's not the same for most people.