There are no bad careers, only bad career matches. A person is generally happiest, I think, when the characteristics of his/her career are a good fit to his/her personality and interests. The careers you've listed - dental tech, graphic designer, real estate agent, and secretary - will all be happy choices for the right persons. But I can't imagine all those careers being a good fit for the same person.
Here are some resources I recommend to people: the first is the Self Directed Search, which is based on what you like to do; it will help you identify the kinds of careers you'd likely be happy in (costs 9.95USD to take it, report is immediately available online):
Welcome to the Self-Directed Search...the world's most widely used career interest inventory!
Once you've identified careers through the SDS, you can read all about them at O*Net Online (enter the DOT codes from the SDS):
O*NET OnLine Help
The second is the book "Do What You Are", which maps your Myers-Briggs personality type to work style preferences and suggests careers that you might be happy with:
Personality Type Welcome - book headquarters for Do What You Are - Nurture By Nature - Just Your Type - The Art of Speedreading People
The third you might take a look at is a free e-book I've authored called "Surviving Career Transitions," which you can download from Scribd:
CareerTransitions
Regarding age, I made a major career change in my mid-50s. If you want it bad enough, you can make it happen. You might find this little page on Late Bloomers inspirational:
Kezi Matthews Late Bloomers Page
Vasily