Yes, they all taste sweet because they activate the same sweet taste receptor. It's also why toxicologists generally become nervous when someone proposes that a collection of structurally unrelated compounds all share the same toxic effect. In the absence of a plausible biological mechanism, an observational association deserves a healthy dose of skepticism. They're a series of unrelated hypotheses involving different compounds, different experimental conditions, different organs, and even different species. Until someone demonstrates how these structurally unrelated compounds could plausibly produce the same effect on the aging brain, I'm not buying it.