Is creatine safe to take for a 16-year-old with a aortic bicuspid valve leak?

No. It can harm kidneys especially when dehydrated and i'm sure you get that way sweating working out! be careful!
Creatine. Since creatine is considered an herbal remedy, it has not had to go through the stringent fda criteria for a prescription medication. Having that been said, use with caution, there are no known toxicity or serious side effects.
Probably. Creatine has been used by a lot of athletes to improve performance. There has been questions about it's use in those with kidney or liver problems, but it appears that it is safe. It's use in persons with allergies is probably not wise. It has not been banned by any athletic organization and in a dose of 5 to 20 grams per day is probably safe. The aortic insufficiency would not be an issue..