To a Younger Coworker: “How old are you?”
This question is a big “no-no” according to any company’s HR policy, regardless if it’s being asked to a younger or older coworker. Your coworkers’ age, along with their race, creed, and sexual preference, is none of your business.
The added emphasis on “are” makes the question extremely condescending, and asking this question to a younger coworker sends the message that you think he is inexperienced or shouldn’t be at the company. So skip the age talk altogether.