It's a secret number so that if Julius Caesar scrambled his message with the key 13, well, then the recipient had better know what that number is or realistically, -- and this is why it's kind of hard to buy -- into some of these age old algorithms, -- what would an alternative approach be to figuring out Julius Caesar's messages to his generals if he didn't know that secret number was 13?

它是一个机密的数字，以至于,如果凯撒大帝使用密钥13加密了它的消息，然后接收者最好知道那个数字是什么,实际上--这就是为什么很难,破解这些古老的算法-,有什么变换方法使凯撒大帝的将军,可以解密凯撒大帝的消息，如果他不知道,那个机密数字是13呢？

哈佛公开课 - 计算机科学课程节选

You can bore people of any age with the wrong approach and you can enthuse people of any age with the correct one.

BBC: Shakespeare 'for five-year-olds'

There is certainly no increase in the accident rate for scheduled air carrier pilots as they approach age 60.

FORBES: Fact and Comment

In fact, as we approach the age of 50, strength or resistance training is crucial for maintaining the ability to have an active and independent lifestyle.

FORBES: Why Strength Training May Help You Live Longer