What are the risks of me getting saline breast implants at the age of 17? going from size a to size b

Not many: The biggest risk is the need to have them removed/replaced many tears down the road. It is a safe procedure done in competent hands.

Risks are well known: Breast implants have been available for over 50 years. The risks are well documented. The most common at the time of surgery: bleeding ; infection. The most common long term: changing size, deflation and capsular contracture (tightening of the scar). Currently, the fda recommends waiting until 18 for cosmetic procedures and any age for congenital problems. Seek a board certified plastic surgeon.