Your basic mental and emotional wiring is in place; you have certain instincts, predilections, fears, and cravings. You have a certain amount of money, certain social connections and opportunities, a certain family lineage. You’ve had a certain amount and quality of education. You’re a certain kind of person.
You are not responsible for any of that stuff; you weren’t yet capable of being responsible. You were just a kid (or worse, a teen). You didn’t choose your genes or your experiences. Both nature and the vast bulk of the nurture that matters happened to you.
And yet when you turn 18, it’s all yours — the whole inheritance, warts and all. By the time you are an autonomous, responsible moral agent, you have effectively been fired out of a cannon, on a particular trajectory. You wake up, morally speaking, midflight.