This look with a blazer and a white and blue crew-neck t-shirt isn't super hard to achieve and is easy to change throughout the day. For something more on the daring side to finish off your getup, add a pair of white low top sneakers. This combo should clear any doubts you had about wearing casual clothes in your forties.
White and Blue Crew-neck T-shirt with Blazer Casual Hot Weather Outfits For Men After 40
This combination of a blazer and a white and blue crew-neck t-shirt is well-executed and yet it's relaxed enough and apt for anything. White athletic shoes add a new flavor to an otherwise all-too-safe ensemble. As you see here, as a middle-aged gentleman, you have a wide range of sartorial options.