The current Recommended Daily Allowance (RDA) for protein intake for adults is 0.8 g/kg body weight per day. This amount is met by most Americans, as it is a small amount. It has been shown, however, that this recommendation is likely too low. Though 0.8 g/kg is probably enough to sustain life functions, it is not enough to support, exercise, growth, a healthy body weight, or the elderly population, let alone to support muscle growth in combination with resistance training. Resistance training causes muscle protein breakdown, which can turnover into muscle protein synthesis, and supplementing with protein helps to build back that muscle and promote its synthesis. While it is true that protein intake has been shown to aid muscle growth, the primary factor is resistance training itself.
One factor contributing to how much protein intake aids in muscle growth is the protein type. It has been shown that whey protein is the most effective form of protein, due to its high content of leucine, an amino acid shown to augment muscle protein synthesis. Casein and soy protein, though good sources of protein, are not as effective for those trying to minimize fat and maximize muscle growth.
Another factor contributing to the effectiveness of muscle gain is the timing of protein ingestion. It has been shown that the muscles are sensitive to amino acids found in dietary protein for 24-48 hours following exercise. This is quite a long period. However, it is commonly recommended that athletes, or those individuals participating in resistance training consume adequate energy within 30 minutes to an hour to maximize recovery and muscle protein synthesis. Further, consuming adequate amount of energy frequently (~4 hours) has been proven to increase maintenance and growth of muscle mass and function.
How does age factor into the mix? Though protein type and timing are important for any individual looking to build muscle, it has been shown that protein supplementation is most effective for elderly individuals. It is recommended that elderly individuals consume about 0.4 grams of protein per kg body weight per meal. For young individuals, 0.25 grams of protein per kilogram per meal is recommended. This difference is simply because muscle naturally is more prone to breakdown in aging individuals unlike young individuals who are growing rapidly.
In summary, no, protein intake alone without exercise does not build muscles. BUT, with the proper timing and types of protein intake, an individual, especially the elderly, can likely increase muscle mass through protein supplementation and a higher protein intake.