For a laid-back and cool ensemble, make a shawl cardigan and dark green sweatpants your outfit choice — these pieces go really well together. Complete your getup with black leather casual boots to change things up a bit. Rest assured, this getup will keep you toasty as well as looking stylish in this summer-to-fall weather. When it comes to casual fashion for 20-something guys, this getup looks cool on almost anyone.