"I'm 40 now. How should my workouts change?"
Good Lord, you're over 40. It's time to hire some Cub Scouts to hold up your stretched out, drawstring-got-lost sweatpants as you attempt to do your quarter-rep, wobbly-kneed squats so they don't fall down around your ankles and expose your graying underwear and God help us anything else that might be graying.
At your age, it's likely you've got dementia and probably horrible, debilitating arthritis, so who can blame you if your pants fall down when you to attempt to squat, you poor, pathetic old codger.
At least that's the kind of advice I feel like giving out when someone talks about "over-40" training.
Look, other than paying attention to your mobility and your endocrine and nutritional status, I don't think there's anything the over-40 guys needs to do that much differently other than getting rid of outdated training fashions like Magic Johnson-era Gold's Gym short shorts or T-shirts that boast about their participation in the 1997 Woonsocket, Rhode Island 5K.
Of course, if you started training in your 30's then "over-40" training might mean something, but if you've been training for at least 10 or 15 years by the time you hit 40, it's just another in a long line of meaningless, arbitrary "milestones" that are based on civilization's obsession with round numbers.
Over-40 training advice needs to be added to the seemingly endless list of "one-size-fits-all advice" like needing to sleep 8 hours a night, drink, I don't know, 16 gallons of water a day, or ejaculate 3.62 times a week for maximal health.
Listen, if you start believing you're over the hill at 40 or 40-plus and that your body has suddenly gotten all snow flakey, then that will become the truth. Instead, rage, rage against this supposed dying of the light and don't bother making any great changes to your training.