at forty, one has no doubts
A yojijukugo derived from Confucius, expressing the idea that by age forty one should have gained enough experience and wisdom to be free from indecision and confusion.
They say at forty one has no doubts, but even at forty I'm still full of indecision.
From the Analects of Confucius (『論語』), where the Master says: 「四十にして惑わず」 (at forty, I had no doubts).