I would say to base the decision on a case by case basis. I could see an experienced, intelligent, and wise beyond their years therapist being helpful even if they are younger. Maybe the odds aren't that great, I'm not sure. If you go one time you should get a pretty good gut feeling.
The number of years working experience a therapist has should make a big difference as well, and most of all whether you think they can help. But with that being said I think people my age are idiots, so I can't fathom working with someone 20 years younger...