A zip sweater and a white and navy crew-neck t-shirt are among the key items in any modern gent's great casual wardrobe. Let your styling sensibilities truly shine by finishing off your ensemble with a pair of white and red leather low top sneakers. As this combination proves, you can't think of a better pick for summertime. When it comes to casual dressing ideas for 20-year-olds, this outfit looks nice on most guys.