Actually we all know as a kid to say "I don't know", but when we become adults we forget that and don't want to say it. That's when we need to rediscover that saying "I don't know" is easy and helpful.

I think you can never ever lie, ever. If you don't know, say, 'I don't know'.
Gore Verbinski

I know.

"It wasn't until late in life" I didn't unerstand how works this structure.... the function of every word. Can someone help me?

William Somerset Maugham CH (/ˈmɔːm/ MAWM; 25 January 1874 – 16 December 1965) was a British playwright, novelist and short story writer. He was among the most popular writers of his era and reputedly the highest paid author during the 1930s.[1]

After losing both his parents by the age of 10, Maugham was raised by a paternal uncle who was emotionally cold. Not wanting to become a lawyer like other men in his family, Maugham eventually trained and qualified as a physician. The initial run of his first novel, Liza of Lambeth (1897), sold out so rapidly that Maugham gave up medicine to write full-time.

During the First World War, he served with the Red Cross and in the ambulance corps, before being recruited in 1916 into the British Secret Intelligence Service, for which he worked in Switzerland and Russia before the October Revolution of 1917. During and after the war, he travelled in India and Southeast Asia; all of these experiences were reflected in later short stories and novels.

To many, it is difficult to say the line, but once one says it, one feels how salubrious it is.

To say "I don't know" sometimes means that you GIVE UP. I think he is suggesting that in his early life, he didn't give up and he worked hard to build a better future for his life.

At the same time we discover how easy it is to say "I don't know", we need to discover the hard way to learn what we didn't know- every time.