For a look that's very easy but can be modified in plenty of different ways, marry a black long sleeve t-shirt with black sweatpants. Get a bit experimental on the shoe front and smarten up this look by slipping into a pair of tan suede chelsea boots. As the weather starts to cool down, you'll see that a look like this is perfect for autumn. When it comes to casual fashion for 20-year-old men, this combo looks nice on most men.