I'm not sure I agree with the message! Some of the things I did in my youth defined who I am today. Some might say it is useless, but what exactly is useless? How do we know what value it will bring in the future? Some things can be useful in one era, and then completely superseded the next.
You are absolutely right. Because not everything will affect everything, especially if someone who will try his best, for example like you, will succeed even at a young age. However, there are others who feel that they have wasted their time and give up without trying again, which is what they are very worried about in the future.