I get asked this question quite a bit. The research is in and if you are already a fit adult at any age, I may add, you may be ready for it. I recommend you working with a certified personal trainer who can safely access when you are ready for this type of training. A researcher by the name of Tabata found that 8 rounds of 20 seconds of bike riding followed by only 10 seconds of rest increases fitness levels, decreases body fat and will even help you lose weight. This study was done with a cardio type of training and not heavy weight lifting like they do in Crossfit. I do not recommend Crossfit to any of my clients because it is unsafe and the injury rate is through the roof. I do believe in pushing past the point of where people push themselves. I have been doing Tabata training with many of my advanced clients for over 5 years, but we do it safely with body weight or cardio training only.