Well, first of all, median family income in the USA for 2015 was $55,775. That varies by state. See
http://www.census.gov/library/visualizations/2016/comm/cb16-158_median_hh_income_map.htmlSo, by definition, half of all families are making less than that. Those families are darned unlikely to be increasing their net worth by $100,000 a year at a young age.
And, of course, only about 74% of US families are making less than $100,000.
https://www.statista.com/statistics/203183/percentage-distribution-of-household-income-in-the-us/ Obviously, it's a lot harder to grow your net worth by $100,000 a year when you make less than that. Still possible, but definitely hard.
Now, for the 26% of US families making more than $100,000, it's a whole lot easier.
So, even if those families have the same annual expense of $30,000, the higher income families will pull ahead a whole lot easier if they act accordingly.
That said, if a family can get a surplus of $12,000 a year, and invest/re-invest it such that they earn, on average, 7% over inflation (the historical US Stock Market return) at a very low fee, they can expect around $1,000,000 at the end of thirty years.
https://www.investor.gov/additional-resources/free-financial-planning-tools/compound-interest-calculatorFrankly, barring illness, legal bills, or some other tragedy, any family routinely making $75,000 or more a year and entering their retirement years dead broke has no one to blame but themselves. Any family routinely making $100,000 or more a year and entering their retirement years without a good sized nest egg has no one to blame but themselves. (For those who are self-employed, "Making $x a year" means your salary and profit, not your gross income before expenses.)
For those who have a family and a family income below the median level, well, they have a
much, much tougher time of it. If they can routinely invest just $300 a month they could expect to have over $300,000 in 30 years. That may not be enough to retire on early, but it's a hell of a lot better than a lot of American families manage.
For most American families, again barring illness, legal bills, or some other tragedy not of their making, a comfortable retirement is a distinct and predictable possibility. For some their income is so low it's very unlikely unless they can get their income up to something in the ballpark of median family income.