I'm taking a wild stab at this and I'm probably wrong: At thirty, we are set in our ways. At forty, we are perplexed. At fifty, we finally know our fate (destiny) [Feel free to attack my interpretation] Another possible interpretation: At thirty, we think we know everything At forty, we are perplexed At fifty, we finally achieve wisdom.

【二節】三十而立。【三節】 四十而不惑。【四節】五 CHAP. IV. 1. The Master said, 'At fifteen, I had my mind bent on learning. 2. 'At thirty, I stood firm. 3. 'At forty, I had no doubts. 4. 'At fifty, I knew the decrees of Heaven. http://www.sacred-texts.com/cfu/cfu.htm