From roughly your thirties onward you lose muscle mass and strength gradually by default, a process called sarcopenia. Left unchecked it accelerates with age, and it's not cosmetic: muscle loss predicts falls, fractures, loss of independence, and mortality in older adults. It also lowers resting energy expenditure and worsens blood-sugar handling, since muscle is where most glucose gets disposed of.
Cardio does not prevent this — only loading the muscles does. Resistance training is the one intervention shown to halt and partly reverse sarcopenia, and it works at any age, including in people in their nineties. Two sessions a week hitting the major movement patterns is enough to make a real difference. The framing that matters: you're not training to look a certain way, you're training so that at 80 you can stand up from a chair unaided and survive a fall.
In 1990 Maria Fiatarone and colleagues at Tufts published a study that overturned the assumption that the very old are too frail to train. They took ten frail nursing-home residents aged 86 to 96 — some using walkers, one with a history of falls — and put them through eight weeks of high-intensity resistance training. Muscle strength increased by an average of 174%, mid-thigh muscle area grew about 9%, and walking speed improved 48%. Two participants no longer needed their canes; one could rise from a chair without using his arms for the first time in years. Fiatarone's follow-up trial in 1994, published in the New England Journal of Medicine with 100 residents, confirmed the findings and showed that nutritional supplementation alone — without exercise — did nothing. The stimulus that mattered was loading the muscle.