Alright, actually, I have got one question like yours today. So, as you said, you are aged over 50. And I believe you cannot wear sunglasses for youngsters right? Thus, you should look for those that are designed for adults. Bear in mind that the color shouldn't be much too vivid and the design shouldn't be too weird, ok? If you really want a pair, you can go to the stores and look for your favorite one in person. Personally, I recommend that you buy a pair of classic sunglasses, that should be ok with you.
Whether is she a fashionable woman? If she is, you can choose the big white frame sunglasses which are the most popular ones these days. This type of sunglasses will match almost all kinds fo complexion. In addition, you can choose the relatively classic black lenses ones which will also look cool.