About : “Oliver Napoleon Hill was an American self-help author. He is best known for his book Think and Grow Rich, which is among the 10 best-selling self-help books of all time. Hill’s works insisted that fervid expectations are essential to improving one’s life. Most of his books were promoted as expounding principles…”
For the quote itself … It is a generalization, many lucky ones have the realization early on (especially the rich and well off). Others, live all their lives in the system and never really make this discovery.
It takes half your life before you discover life is a do-it-yourself project
Assuming we are talking about the general situation, indeed it takes us a long time to realize that life is a project that is different to each. We spend a big part of this life in the education system and then in the labor market. We get used to living in the system and following the “best practices”, in reality each can shape life, the way they see fit.
What Napoleon Hill here is basically pin-pointing out to is mid-life crisis. The moment when people realize that time flies fast, and that they did not enjoy life as much as they should, or have not achieved goals worth achieving.