Depends where you live. Some places I've lived, people would struggle to live off of $20 an hour. Other places, you'd be one of the wealthiest people in the city if you made $30 an hour.

It varies drastically. Where I live, in a moderately-sized city, the hourly living wage—according to MIT—is listed as $21.13 ($34.25 in NZ). Quite frankly, I think that's much too low, considering price gouging and the astronomical health care costs in the US. I've made that much, but it wasn't even enough to add to my savings, and I certainly would've been fucked if I got seriously sick... even with insurance.

The minimum wage here (and federally, though many states have increased their own minimum wages) has been $7.25 ($11.75 NZ) for 15 years... and it was too low back then.

No one can survive on that wage anywhere, at least unless you're not paying for housing (and even then, I have serious doubts). Conveniently, the amount listed as a "poverty wage" is $7.24.