To me, even as someone who comes from a pretty solid middle class background, this seems kind of difficult to do honestly unless you have zero debt + a spouse/partner to offset expenses + well paying jobs with good benefits + start your career early, with minimal setbacks and good compensation.
I actually just signed up to start contributing to my current job's 401k, but I've been here two years. I'm contributing 8% of each paycheck, but I wish I could do more. I should have started immediately after becoming eligible (6 months of employment), but neglected to because the company stopped matching contributions right as I became eligible (dumb of me).
I'm 27, so the thought of getting to double my salary right now by 35 doesn't really seem feasible. I'm trying to pay down student loans, of which I still owe over 15k, and pay off my car (but it's 10 years old and I'm probably going to wind up needing a new one sooner than later, so more debt yay!).
I lived with a girlfriend the last two years so living expenses were halved for me, but now we've separated and I'm trying to figure out a decent living budget that covers all my expenses (housing+utilities, loans, investments, food, etc.). If the rough average of ~7% interest gained on your standard market investment plan, and you were contributing 10% every paycheck, it seems like it'd still take more than 10 years to get to double your salary...