It is in part. A person on a US$20 million a year salary obviously isn’t working class or even middle class-they might truthfully say they are of “working/middle class background” (if their parents were of more modest means)

Whereas, their 10 year old kid whose annual income and assets are roughly $0 is also upper class-because their parents are. Now, if they grow up and their parents end up disowning them and they end up living on the streets homeless-then they won’t be upper class any more. But if that doesn’t happen, then they are-because even if they aren’t filthy rich yet, very likely they’ll eventually inherit a huge amount of money from their parents (assuming they don’t already have some kind of “you’ll never have to work a day in your life” trust fund set up for them). They could live their whole life in the upper class without ever getting a salary at all