With aging you can expect to lose muscle size and strength, which increases risk for lifestyle diseases and disabilities (GeroScience, 2020;42:1547–1578), such as:
• heart attacks (Eur Geriatric Med, 2016;7(3):220–3)
• diabetes (Med J Aust, 2016;205(7):329–33)
• osteoporosis and fractures (Arch Endocrinol Metab, 2015;59(1):59–65; J Gerontol A Biol Sci Med Sci, 2018;73(9):1199–204)
• depression and memory loss (GeroScience, 2020;42:1547–1578)
• loss of physical independence (J Nutr Health Aging, 2020;24(3):339–45; 2019;23(2):128–37)

Progressive loss of muscle starts at about 25 years of age. It is caused mainly by a loss of muscle fibers and to a lesser extent by a reduction in type 2 strength fast twitch fibers (J of the Neur Sci, April 1, 1988;84(2-3):275-294).

Why You Lose Muscle with Aging
Every muscle in your body is made up of thousands of muscle fibers just as a rope is made up of many strands. Every muscle fiber is innervated by a single nerve. With aging you lose nerves, and when you lose a nerve attached to a muscle fiber, that muscle fiber is lost also. A 20-year-old person may have 800,000 muscle fibers in the vastus lateralis muscle in the front of his upper leg, but by age 60, that muscle would probably have only about 250,000 fibers. For a 60-year-old to have the same strength as a 20-year-old, the average muscle fiber needs to be three times as strong as the 20-year-old’s muscle fibers. You cannot stop the loss of the number of muscle fibers with aging, but you certainly can enlarge each muscle fiber and slow down the loss of strength by exercising muscles against progressive resistance, using strength-training machines or by lifting weights (Experimental Gerontology, August 13, 2013).

How to Strengthen Muscles
To enlarge and strengthen muscles, you need to exercise them against resistance forceful enough to damage the muscle fibers. You will know that you have done this because you will feel a burning in the stressed muscle during the later lifts and on the next day, your muscles will feel sore. Then you lift lighter weights, or engage in other sports, for as many days as it takes for your muscles to heal. You can tell this has happened when the soreness goes away. When your muscles feel fresh again, try to lift weights a few times in a row or lighter weights many times in a row. You can become quite strong by using 10 to 15 strength-training machines or exercises (for different muscle groups) three times a week. Always stop immediately if you feel any pain, tearing or excessive burning. See Making Muscles Stronger.

Inactivity Accelerates Loss of Strength
When muscles of young people are immobilized for two weeks, they lose strength equivalent to that of people who are 40 or 50 years old. As soon as you stop moving, your muscles start to weaken and the larger your muscles, the more muscle you have to lose. Younger and stronger people lose strength faster than older and weaker people (J of Rehab Med, June, 26, 2015).

Even if you already do resistance exercise, realize that taking off for a long period will cause significant loss of muscle size and strength. People who lift weights and then stop lifting lose muscle size much faster than those who never lifted weights. After just one week of being bedridden, you can lose as much as two pounds of muscle (Nutr Rev, Apr 2013;71(4):195-208). Even if you are not immobilized, you can lose about 11 percent of your muscle size after ten days without exercise (Scand J Med Sci Sports, 2011 Apr;21(2):215-234).

A weightlifter can lose up to 20 percent of muscle size after taking a week off from lifting, but this loss of muscle size is not due primarily to loss of muscle fibers; the primary cause is loss of water and glycogen (sugar) stored in muscles (Scand J Med Sci Sports, 1999 Aug;9(4):209-13; Eur J Clin Nutr, 1999 Feb;53(2):126-33.) When a weight lifter resumes lifting, the sugar and water can refill to their former levels in muscles quite quickly (Physiol, Apr 2013;113(4):975-85). Weightlifters who are fully immobilized for two weeks by an injury or illness will probably take more than six weeks to regain full muscle strength (J Rehabil Med, 2015 Jun;47(6):552-60).

My Recommendations
I believe that every healthy person should have a progressive resistance program as part of their regular exercise program. It can help to stave off disability and disease. If you have never lifted weights before, you should first check with your doctor for any potential problems and in the beginning, seek out instructions on progressive resistance training. See Weight Training for Middle-Aged and Older People
Resistance Exercise You Can Do at Home