A photo of yourself would have been helpful to give you a better answer. However the implant size you've selected sounds reasonable.
Than you for the question, Silicone implants are approved for woman above the age of 22. If the size is chosen based on the individuals tissue characteristics the chance of having a great long term success is high!
Thank you for your question. It is very helpful to have a picture of where you are starting from to help determine what is adequate. Adding a 339cc moderate plus silicone implant is a very reasonable size and will likely place you into a C or D cup range, depending on your chest size and how much breast tissue you have. More information is needed, but with what you have presented, your surgeon is heading you in the right direction. Make sure to clearly state your goals to your surgeon and trust that they will help achieve them for you.
Best of luck.
You did not include photos of your own preop anatomy so no way to tell if your goal is realistic. If you have no droop, then the goal is probably realistic, but we cannot ever comment on specific implant sizes without an exam.