If you’re looking for a style that emulates youth and vitality, Wayfarers are the way to go. These have been around for decades, but they keep coming back for younger people (although the older generation can enjoy them too).
The sunglasses have symmetrical frames that are flat on top and rounded at the bottom. They most often come in black on black, but there are plenty of other styles available in all colors. There are variations on the Wayfarer as well, that provide a slightly less rounded lens on the bottom half.
Wayfarers have been popularized by many different manufacturers, but Ray-Ban is probably most well known for creating them.