You say you get thinner as you get older. If you don't give up halfway, you must be undernourished. You should practice three-point muscles and eat seven-point meals. You should add more protein and eat more chicken and beef. Also, the weight of exercise should be chosen according to your own situation, and it should be gradually increased every once in a while, so as to stimulate the muscles enough. The most direct thing is to see if the muscles will feel sore the next day after exercise. If there is pain, muscle exercise is effective. Exercise this muscle at least every 48 hours to give it a full rest.