Wear a black blazer with grey jeans to assemble a truly stylish and modern-looking off-duty outfit. To add a more casual touch to this outfit, throw a pair of slip-on sneakers in the mix. When leaves are falling down and fall is settling in, you'll love this outfit as your go-to for in-between weather. This ensemble suggests how to keep slaying in the casual fashion department as a 30-year-old gentleman.