We lose muscle mass and strength as we age. This is known as sarcopenia.
Muscle loss could begin in our 30s. Genetics and lifestyle play a major factor and there is a lot of inherited individual variability
There are several reasons why we become weaker: Our nervous system becomes more inefficient, our muscle quality decreases, our anabolic hormone secretion decreases, our bodies develop chronic inflammation and anabolic resistance.
How to prevent age-related muscle loss: eat more protein – at least 1.2g/kg BW up to 2g+/kg (counter-indication: careful if you have damaged kidneys), strength training with high intensities and volumes, active lifestyle outside the gym, supplement various nutrients like vitamin E, D, and omega-3s (if you are deficient).
There are also riskier solutions, but for those you have to see your doctor.