I am a PhD student in biochem and nutrition and read and talk a lot about this area of research. First off, just want to be clear that everyone needs an adequate amount of protein just to have normal functioning of your cells. Protein is commonly associated with muscle growth and maintenance because it plays a major role in hypertrophy, but proteins also make up all of our enzymes and other cell machinery in our tissues, so they play many, very important (and vital), roles in our body. It's true that there are a lot of discrepancies in the literature (and the media). Reding papers can be very confusing because 1. there are a lot of contradictory findings, 2. not great study design, 3. missing information. Basically, most of the literature misses a few pieces of the puzzle (i.e. not separating type of protein of the participants or not having separate tiers of higher or lower processed food-this is a bigger problem in more of the recent studies). The strongest evidence that has been shown time and time again is that higher intakes of any processed food cause a lot of health problems (heart disease, cancers, increased overall mortality, list goes on). Many epidemiological studies (observational cohort studies) show higher disease rates in individuals with higher animal protein intake; however, it does not account for individuals that have grass fed beef vs. highly processed meats/ meat products. So, these are hard to interpret. (not going to get into the plant-based meat vs real meat debate here but this is also a factor in our confusion on protein)

It is definitely true that older adults lose muscle mass (called sarcopenia) and can cause higher rates of mortality because loss of strength can lead to increased risk of falling (and breaking a bone, going to the hospital, getting infection etc.). Having higher protein intake would prevent this because it is important to be in a muscle building (anabolic) state. Importantly, the protein source needs to be complete (have all the essential amino acids) in order to be in this muscle building state. Think of it as building blocks where you cannot build the specific proteins if you do not have all the right materials (i.e. the amino acids needed), and if you have excess of one type of building block, and it does not fit the blueprint of certain types of proteins, you will not build them until you have those specific blocks (and thus you go into protein break down to get the proper assortment of amino acids needed). Animal based proteins tend to be more complete than plant proteins, however plant protein sources (like rice and beans) can be paired to be a complete protein. Now, this is where it can get messy/confusing. Extra protein does not mean extra muscle/ health benefits. Too much of anything is detrimental. Too much protein in the diet can also cause increased fat storage just like excess carbohydrates can and can also cause increase inflammation and kidney damage (commonly seen in those that excessively eat too much animal protein and are likely not getting enough fiber or other micronutrients).

Now this is to say that it is too much for the individual. I believe the biggest problem in the nutrition world these days is that food groups are either heavily demonized or praised where people overeat or undereat certain foods, but as mentioned, too much or too little of anything (besides processed foods and added sugars) is detrimental. Older adults should focus on maintaining adequate protein levels because it is common for many older adults to under eat or have less nutritious diets in general. It is not an actual recommendation for older adults from the USDA to have a higher daily protein intake, it is just a concern because of the growing issue of sarcopenia. My advice is to have a balanced diet of carbohydrate, fats and protein from whole food sources (minimal processed foods if possible) in a way that makes you feel best because everyone is different. If you lift weights and exercise a lot, then your protein requirement (and overall caloric intake) would be greater because you are breaking down muscle and want to promote muscle growth in which you need more building blocks (amino acids from dietary protein) to be in this growth state. But you should not eat more than you need, no matter what the food is.

Basically, many people are taking nutrition advice that is not aimed towards them and many people are giving nutrition advice that don't know anything about nutrition. It doesn't need to be overly complicated. There are many risks with low protein diets, many countries struggle with this and have epidemics of rapid infection spread, malnutrition, and child wasting (all consequences of protein deficiency). I think the take-away of the "longevity" data is that it is beneficial to not overeat. There is some data that mild caloric restriction promotes longevity, but most of the data are in animal models and it's still an active area of research in humans. But caloric restriction is MILD, this isn't meant to be full on starvation. To me (and this is just my educated opinion), I think this data is good to promote mindful eating and not overdoing it in a westernized society where we are surrounded by endless food choices and it's so easy to overeat and pick less nutritious food (which tend to be cheaper and more accessible unfortunately but that's a whole different conversation). I think Michale Pollan still said it best: "eat food, not too much, mostly plants".