Most of the advice in "How to Win Friends and Influence People" is fairly relevant and can be learned at any age - the book and Carnegie's courses were originally for older businessmen. The problem with the book is that its title and chapter titles have not aged well - they sound manipulative. The actual advice is very good and focuses on empathy and kindness; timeless concepts.

I've definitely seem people change their behavior after reading it well into their 30s and 40s.