18. 18 for saline 22 for silicone.
18. If you are under 18, there is a possibility you can have your parents sign for you but silicone implants can only be used if you are over 22.
18 in most states. Surgical consents require that the patient be an adult (unless a legal guardian signs) which in most states is 18. The american society of plastic surgeons does not recommend that pure cosmetic procedures be performed in minors except in special circumstances.
Age. There is no legal age depends on situation if patient has poland syndrome then they may feel uncomfortable that is up to the parent. I have performed breast augment on a few 17 year old.