Billionaire Warren Buffett is viewed as one of the world’s greatest investors, so his annual letters to Berkshire Hathaway shareholders are always read closely for lessons.

The 94-year-old Buffett also acknowledged his advanced age in his annual letter, telling shareholders he now uses a cane and will spend less time fielding their questions at Berkshire's annual meeting in May.

Warren Buffett started what would become the most profitable investment in his legendary career. Through his company, Berkshire Hathaway Corp. (BRK.A), he began accumulating shares in Apple Inc. (AAPL),