It's an impression based on the people I have worked with over thirty plus years in the theatre. I have never kept official detailed stats, but I'd guess that at least 2/3's of my collagues are from middle- to upper-middle class backgrounds. Parents are almost uniformly professionals and executives and educators. I know very few whose parents were working class or grew up in shaky circumstances.
My parents both had very low-paying office jobs and were helped by a handful of single working aunts who lived simple lives and spent little. It was not a deprived background compared to many but in the theatre world, I often feel particularly impecunious.