Middle class is a pretty subjective term. I’ve known people who made a half million dollars and chose to live in ancient house trailers and dove old beater pickup trucks.
Between $40 and $50 grand for a single person is what I consider to be middle class depending on things like location and lifestyle.
I considered myself “middle class” when I made $30K (in the ‘90s). I think it’s more a state of mind than a defined income range, as well as how you manage your income.