I'd say primarily hormone driven, child or adult. The difference in effects of resistance exercise between child and adult are hormone driven....

Maybe protein limited rather than protein driven? You can take somebody eating lowish protein, add exercise, and it results in a more positive nitrogen balance, as long as things aren't overdone. Adding protein might make the nitrogen balance more positive still--but might be ineffective without the exercise.

I guess everything depends on what you call the baseline. I'd agree some fruitarian eating 20 grams of protein a day probably has their muscle mass limited by protein rather than exercise or even hormone status, but get into what most of us here probably consider lowish--the standard recommendations--and already resistance exercise might be the more important intervention, versus upping protein further.

This sort of reminds me of the calcium thing. We've got calcium recommendations over a gram, supposed to protect people from osteoporosis, meanwhile we've got people eating a third of that in a lot of places, without osteoporosis being a problem. In that case as well--exercise is a big deal.

A plug for vitamin D probably fits in here somewhere as well.