For something more on the casual and cool side, go for a navy print short sleeve shirt and navy chinos. Look at how great this getup goes with dark brown suede driving shoes. This outfit should erase any doubts you had about wearing casual clothes over 40.
If you're looking for an off-duty and at the same time dapper outfit, dress in a navy print short sleeve shirt and navy chinos. As for shoes, introduce white leather low top sneakers to this outfit. Those who are curious how to dress fashionably as you move into your 40s, you have your answer.