Researchers call it anabolic resistance: with age, the same serving of protein produces a smaller muscle-building response than it used to. It's normal, it's well documented, and it isn't a disease. It's just a dial that's been turned down.
You don't feel it as a number. You feel it as soreness that lasts three days instead of one, and as sessions that used to be easy feeling harder to recover from, even though you haven't changed what you eat or how you train.
The practical answer in the research is not simply more protein. It's a stronger, faster signal around training: more of the nine, arriving sooner. That's the whole idea behind a free-form top-up.
Sources: International Society of Sports Nutrition position stand on essential amino acid supplementation, 2023 (doi:10.1080/15502783.2023.2263409); Paddon-Jones et al., amino acid ingestion and muscle protein synthesis in young and older adults (PubMed 14583440). These studies examined essential amino acids as a category, not Bare Aminos.