Dress in a black crew-neck sweater and a white crew-neck t-shirt if you seek to look cool and casual without too much effort. Make black suede desert boots your footwear choice for a dash of refinement. Seeing as the weather is getting colder with each day, this look is a viable pick for the transitional season. As you can see here, as a 20-year-old gentleman, you have a vast array of style options.