Consider pairing a navy crew-neck sweater with navy plaid chinos to put together a casual and cool look. A pair of white print leather low top sneakers is a good option to finish off your ensemble. This pairing should erase any doubts you had about wearing casual clothes as a 30-year-old gentleman.
For a relaxed look with a modern take, you can rely on a navy crew-neck sweater and navy plaid chinos. Feeling inventive today? Change things up a bit by finishing with dark brown leather loafers. As a guy in his thirties, you probably want to start dressing like an adult. If that's the case, looks like this become very relevant.