Gotta disagree with this. As someone who grew up middle class, I was fortunate, in a lot of ways, to also learn to be prudent with money. I started bootstrapping microbusinesses with almost no monetary investment. This eventually allowed me to support myself (on a very low budget), and then, later, on a slightly larger budget. Eventually, one of these businesses became successful enough to support a family comfortably with an "upper" middle class income.
The mistake a lot of people make is that they need to spend significant time and/or money to start a business. You don't. You can work on low-risk businesses, building your skillset so that your odds of success improve over time. No need to aim for the "center" bullseye, either. This is a case where low ambition can be helpful. Instead of trying to grow a huge startup, I was ecstatic when I was able to afford rent in my cheap apartment (with a roommate.)
Of course, the situation may be different now for middle class kids. Maybe they don't have some of the same luxuries my family was able to give me: a couple of years of college paid for, attending a few business/marketing conferences as a teenager, examples to follow from my dad and brother. I suspect some of this is true; I sense a lot of pressure on young adults that I didn't feel when I was young. I also recognize that "middle class" in the states is definitely a case of "wealth" in comparison to other parts of the world.
Also, when I was young, and learning the ropes, I was young. I didn't have responsibilities; just being able to afford a crappy apartment and food was more than enough.