Nothing about your 20s is straightforward. And neither is this book.
You can pick it up when you’re 20, and answer all 20 something questions, year after year. Or, just answer the ones that resonate.
You can pick it up in your mid 20s, and reflect from then on forward. You can go back, and reflect on your earlier years.
You can pick it up when you’re 18. I think that’s what my precocious younger self might have done.
OR you can do what I’m doing now - start at 29; the end of the 20s era.