Is it legal if a potential employer asks me during an interview how old I was?

I was flown in for an interview from my home in Idaho to the company headquarters in Colorado. I am 51 years old and have been told by many people I look like I'm in my 40s. The interviewer was talking about what it took to be a successful sales rep and in his experience older reps did not have the drive of younger reps who wee trying to make a name for themselves. He then asked me how old I was. I have not heard back from this potential employer this was 2 weeks ago.