About one in five Americans works past the traditional retirement age, according to Pew Research. And billionaire Warren Buffett has proven that it can be quite lucrative to keep working in older age — the 94-year-old Berkshire Hathaway CEO earned close to 95% of his personal wealth after age 65, per Barron's.

Warren Buffett says he'll keep penning letters, and speed up his giving but retain a big Berkshire stake until his successor wins over shareholders.

The conglomerate sold $12.5 billion of stock in the latest period and bought $6.4 billion, marking the 12th consecutive quarter of net selling.