Imagine if you could grab hold of every 6 year old on their first day of school and force them to put away 10% of every single week's pocket money and/or money from doing jobs etc. Then tell them that they will retire at 40 as a very well-off individual, and they would believe you and do as you say..
I believe over-promising the benefits of a particular course of action can be harmful, and the above claim is fanciful unless the child was spoilt rotten with pocket money (so they could get fantastic compound returns from early on).
Even with the magic of constant reinvestment of compound returns, the above recipe would only get (say) a deposit by the person's mid-20s, and maybe outright ownership of a modest home by their 30s (especially if they have a partner with similar discipline).
They would be comfortable but not financially independent, and nowhere near being able to retire.
Assume there is zero inflation and the following incomes:
$10pw pocket money between age 6 and 10 (total $2500 of which $250 would be saved)
$20pw pocket money between age 11 and 16 (total $5000 of which $500 would be saved)
$200pw part-time earnings between age 16 and 21 (total $50 000 of which $5000 would be saved)
$1000pw full time earnings between age 22 and 40 (total $900k of which $90k would be saved)
Account for conservative investing and compound interest and the total saved might be somewhere around $300 - 500k. A fine achievement by age 40 but far below any definition of 'very well off'.
So following that above instruction is an excellent foundation but still an over-promise.
Nope, to be well-off you'd need either:
* Saving a much higher proportion than 10% of income - think 30% plus
* A very high income and thus higher proportional savings (if you can restrain spending)
* Some element of increased risk to raise returns, eg borrowing money to leverage investments, starting a business or high risk/insecure but lucrative employment
So maybe it's better to say that a 10% saving plan will protect you from abject poverty, having to beg and serious need. It will also gives a reasonable chance to buy your own home, and if continued, supplementary investment income over the age pension. But by itself it won't make you rich and shouldn't be promised as such.