Most people don't understand how important, even vital, proper rest is. Injuries are almost always the result of overuse and/or insufficient rest.

HIIT itself is not dangerous, or even bad for you, if you spend the time to learn about it and how to incorporate into your life safely. If you're out of shape, and don't train regularly, go out and try HIIT you're going to have a bad time.

yeah this is what I meant - as we get older we're supposed to move slower, and spend more time working on mobility and strength (moderate strength building, walking, and other movements like taichi and equivalent)