Can soy milk's phytoestrogens stunt the growth of an adolescent during puberty, by lowering testosterone which prevents growth spurts, and closing growth plates early?

Don't think so: There are people all over the world who eat soybeans and their products as a big part of their normal diet, and we don't see any outbreak of stunted teens. The key is to have variety in one's diet, so not to eat the same things every day, unless those same things have ingredients from a variety of food types. Variety helps assure balanced nutrition.