To maintain muscle mass and strength in older adults, research generally recommends 1.0 to 1.2 grams of protein per kilogram of body weight per day, spread across multiple meals. Resistance training is the most important factor; protein intake enhances the effect but has little impact without training. Functional gains in daily life can be noticed after just a few weeks, even when the increase in muscle mass remains modest.
Strength training is the most powerful lever you have. Multiple well-designed studies show that it clearly improves muscle strength in older adults and makes everyday movements such as climbing stairs and getting up from a chair noticeably easier. That effect is well documented and consistent enough to serve as a starting point, even though the optimal dose differs from person to person and depends on fitness level.
Protein intake reinforces that effect, but barely works without the training alongside it. The amount most commonly used in research for older adults is around 1.0 to 1.2 grams per kilogram of body weight per day, spread across meals rather than all at once. At 70 kilograms that works out to roughly 70 to 85 grams per day, obtainable from everyday foods: eggs, dairy, fish, meat or legumes. A protein powder is only useful if you cannot easily reach that amount through food. Caution is only relevant in the case of an existing kidney condition.
The gains in muscle mass visible on the scales or a scan are often modest in older adults, even when strength increases substantially. It is therefore not just about muscle volume, but about what you do with it in daily life. The latter improves for many people after just a few weeks of training.
Overview covering multiple factors (2 research records, 4 sources). The strength of evidence differs by component -- read the answer for the nuance.