I first read the Millionaire Next Door as an 8th grader (20+ years ago). I then read the Millionaire Mind and followed Stanley's blog up until his passing. At that time, the personal finance space as it exists today simply didn't exist back then. TMND was the first personal finance book I read and it had a profound impact on me. As a 13 year old, I learned about the importance of career, living below your means, the concept of all hat and no cattle, the age and demographic of most millionaires (e.g. correlation between age and wealth) which meant I might need to be patient in accumulating wealth, importance of choosing a spouse, the fact that millionaires weren't necessarily straight A students, concept of keeping up with the Jones, over representation of business owners, examples of people in ordinary careers (i.e. teachers) becoming millionaires, and how business owners in blue collar or unsexy fields could become financially successful. I couldn't think of a better book to start off my financial education.
While there are way more options today for financial education, there is also way more noise. Imagine being a beginner and starting your journey by reading Wall Street Bets on Reddit or another similar message board. You might be tempted to throw all your money into a Meme stock or crypto. Or you might see peers right out of college driving entry level German cars. TMND helped me see past that noise. I don't know anyone else my age that is reading E-R, Bogleheads, or really any books on investing or personal finance. Most of my peers just ask their parents for financial advice such as how to invest their 401k.