I know once when I called Alpha Men’s Spa to ask who’s available, he voluntarily replied “we have one Asian, one black, and one latino available right now.” (I’ve tried each of them since their opening). Generally, None of these spas really seem offended if you ask the masseurs’ ethnicity as they know everyone has a preference. In terms of diversity, my personal experience is that Spa27, Shangrila, Top Men, Super Men and Utopia have the most diverse set of masseurs. (I haven’t been to the last two in over two years, but I assume it’s the same).
I’ve never inquired about the ages of masseurs, but they’ll probably answer that for you as well.