How old do you think someone should be before they think 'Hey I'm too old to start a new career'?

It's a tricky subject because I guess it depends on the degree of change. 

Perhaps there are ways to turn things dramatically around for those who have the drive and energy. 

Are you one of those people?   If so, we would like to hear from you. 

Or are you over 50 and pondering changing your career?