I have an issue which makes my aorta weaker than normal and they were afraid that as I grew up that with normal development it may end up too big/weak and could be dangerous for me. They didn't want to risk aggravating it. I'm an adult now though and it's remained within normal range/not changed at all since I started going to the cardio so they feel okay with me being able to get normal exercise again as long as it's not competitive and I am able to control my own stress lever/can take a break when I feel I need to.