Choose a grey long sleeve shirt and navy jeans to achieve an interesting and current relaxed ensemble. Send this ensemble a more relaxed path by rounding off with grey athletic shoes. As you can see here, as a mature guy, you have a wide range of dressing options.
Consider wearing a grey long sleeve shirt and black chinos for an everyday look that's full of charm and character. For something more on the daring side to finish off your ensemble, introduce a pair of grey athletic shoes to the mix. This combo demonstrates that after 40 you still have a wide range of dressing options.