You're saying that because most people make more money later in their career that the US doesn't have an issue with upward social mobility.
No, that's not what I'm saying at all. But you seem determined to intentionally misinterpret what I'm saying, and this conversation isn't doing anything to benefit me, so I'm bowing out.
The fact is, most people end up in the same socioeconomic class as their parents. There are exceptions to the rule, but upward social mobility happens so infrequently in the US, that it's perfectly reasonable to model that as almost never happening. It's not impossible, it just almost never happens. The bigger problem is that the few counterexamples are used as anecdotal evidence to say "if everyone else would have just tried as hard, they'd be better off too". That's simply not true.