Several years ago in Palm Springs, CA, there was an instructor by the name of Bill, and at the tender age of 89, he was still going strong. My point is this: Live your life with NO regrets. If you are interested in teaching group fitness classes or personal training, seek the education in which you need and go for it. I would highly recommend anyone taking classes for at least six months, to see what format you enjoy such as step, kickboxing, dance based classes, strength, Zumba, etc.
Do your research to see which certification you wish to hold. I will list some sources below. Some companies you can do online, while others you have to physically be there. There are also many conventions in which you can attend for a weekend to get all the training you need at once. Do not live your life wishing, I wish I would have when you CAN!
Here are a few sources:
Aerobics and Fitness Association of America
American Council on Exercise
American College of Sports Medicine