Breast Augmentation: In front or Behind the muscle
It just all depends on your breasts. It really has nothing to do with the age. You may be fine with submuscular or you may need a dual plane (type II or III). I generally dont ever go in front of the muscle. If there is a need for subglandular implants, meaning the skin needs some filling and lifting, then a dual plane which uses the advantage of both subglandular and submuscular position would work well.
Breast implants in a 64 year old
The key issues are the amount of sag (ptosis) as well as the amount of tissue and your overall health. Pending medical clearance and a full aesthetic consultation breast augmentation is an option.
You certainly can get a small implant and a lift in most cases. Of course you need an exam first during a consultation.
Implants over the muscle are better to correct any sagging.
Your age is not an issue. Of course, the position of the breast implants is chosen after examining you, but, in general, if there is any sagging to improve, you will look better with implants over the muscle.
Submuscular breast augmentation
Provided you are in good health and don't use tobacco products, breast augmentation with or without a lift (mastopexy) may be an excellent option for you. Submuscular implant placement sounds very appropriate, as it will camouflage the implant more. Given your lean build, and the probable loss of tissue from the upper pole of the breasts, a silicone gel implant may be a better choice, as it will demonstrate less visible rippling.
You'll benefit from an evaluation by a board-certified plastic surgeon. Best of luck.
Breast Implants at 64 years old
Age per se is not a contraindication, If you are in good health and you have met with your plastic surgeon to determine yours goals and expectations and to review the risks, then you should have no significant increase in risks. You appear to be realistic in size goals. Your surgeon will evaluate your skin with the "pinch" test to determine if a subglandular or submuscular place may benefit you best. You will get a slight correction of ptosis (drooping) just with the augmentation,but i more needs to be done there are alternatives. Good Luck!
Consider silicone implants
If you have a lean build and not much breast tissue, you may want to also consider silicone implants. They are somewhat more like breast tissue, so in a very lean patient they feel slightly more "natural." Discuss both saline and silicone with your doctor. I think submuscular sounds like a good plan. Make sure you are in good health and have your primary care doctor see you before surgery.
Submuscular is probably the best pocket
You state you are lean and have very little breast so I would suspect the submuscular pocket would give you the best results. You can discuss the pros and cons with your surgeon but it is generally better to have more tissue thickness over the implant; no matter what your age is.
You need to be seen and evaluated.
It sounds like you need a small implant and a breast lift ( mastopexy). Exactly where the implant is placed is up to the plastic surgeon and the amout of droppiness you have has to be evaluated.
Things can be done and your age should not be a factor as long as you are healthy.
Breast Augmentation for a slight lift
Breast implants are a perfect solution for an individual that has a good overall shape -- but slight droop from breast atrophy. This is usually secondary to pregnancy, weight loss, or age. Refilling the envelope with a submuscular saline or silicone implant will make you happy and feel better about your self.