First of all, I want to say the excessive ultraviolet rays in light would accelerate the occurrence and development of senile cataract and Senile chloasma. Wearing sunglasses is beneficial for both the young and the senior.
When choosing sunglasses for elders, I have some tips for you. Firstly, choose the lens that can prevent ultraviolet rays, secondly, choose some light color such as grey, tawny etc. because the dark color can make the environment looks darker when wearing them, and especially for the old people who are less attentive thirdly, if the users have vision problem, you had better choose the lens that have spectacles.
The older people need the sunglasses most because the eyes protection is very important for the aging people who will easily get the eye diseases. You can buy the ray ban sunglasses for your parents to make them look cool. And that brand of sunglasses will be excellent in quality and design.
Looks like you are going to buy your parents visual aids right? So, as far as I am concerned, what would mostly be suitable for the elderly is traditional square shaped glasses. Since that kind of glass frame is accepted by the general public, so your parents could feel free to wear them wherever they go, I mean, by wearing these glasses, they are more like an American.
It is hard to see what kind of sunglasses are the best. It depends on what you want to get from sunglasses. If you have no special needs, just pick up sunglasses that your parent may like it according to your parents' value of beauty. If your parents often drive out, play golf or go to fish etc, you need to buy polarized sunglasses to them. Hope this can help you.