To be honest, all this "what happened in your childhood does not matter, because when you are 18 years old, your life resets to tabula rasa anyway" always felt very suspicious to me. (Strawmanning to better express how I feel about it.)

The employment you have while living with your parents as a teenager is a rounding error on your lifetime income

Sure. But the money your parents have, and how much they are willing to support you, influences whether you have to take the first available job to pay your bills and then get stuck in the "rat race", or whether your hands are untied and you can experiment with learning and working on your own projects.

education past middle school is almost entirely about signalling

Maybe I had an exceptionally good university education, but I believe that learning computer science helped me understand the concepts that I probably would struggle with otherwise; at least judging by my colleagues who are smart and diligent, but didn't have the same education.

Also, I'd bet that social class strongly correlates with parents' social class. But yeah, one could argue that the social class is determined by genes. So we would have to study adopted children.