There is no upper age limit for breast augmentation. However, a 68-year-old woman will likely have some breast ptosis and will likely also benefit from a breast lift ("mastopexy") as well, in order to have the best cosmetic result.
A breast lift will remove excess baggy skin, lift the nipple and areola up to a position on the breast which is aesthetically pleasing (but age-appropriate). An implant will add back volume that may have been lost over time. The results should be as natural as possible.
Surgery is not for everyone; some women choose instead to invest in a great padded and push-up bra to achieve the same results non-surgically.
To learn more about your surgical options, be sure to see a board-certified plastic surgeon with experience in breast surgery, particularly in older women, and who has experience in combination mastopexy and augmentation. See a number of experts until you find the very best fit for you.