Take it from someone that is 48yo and have been able to get my belly to shrink back to flatter than when I was younger.
Pay attention to diet/nutrition the older you get. The older you get, the more important it is to balance your testosterone, cortisol, estrogen levels. Avg males can lose up to 50% of their testosterone levels by the time they are 50yo. Testosterone starts going down for most starting at 30yo.
Find out the foods to stay away from and foods to eat that increases testosterone levels. There's no shortage of videos that recommend these types of foods on youtube.
I won't get into details and bore everyone but generally laying off the alcohol, processed foods, high fructose corn syrup(soda), processed carbs(rice, potatoes are ok), and other fatty foods will do wonders.
For me personally, it's been the nutrition and Reverse gravity situps(leg lifts) and bycycle situps that's really flattened my stomach. Before that, no amount of regular situps or incline situps would work. Obviously everyone's body is different so you gotta try what works for you.