Lore has it that SP500 doubles your money every 7 years. If someone is 60 and just started, well, it's not going too high.
But for someone who is 20something and begins placing $€200 per month in SP500 (preferably somewhere with the lowest possible fees), and does so every month for all the years he/she works, then there is a very nice surprise waiting for them (and their kids) later in life.
Keep in mind, investment funds don't die like our pensions, they are transferred 'down'. So even if someone has e.g. 200k when they have kids, by the time those kids turn 21, that 200k would have turned to 0-7yo 200k->400k, 7-14yo 400k->800k, 14-21yo 800k->1600k. It needs discipline and consistency though.