Instead, I tell them to invest the amount they won't be needing for the next 5+ years, this prepares them mentally and physically, and those that are weak will give an answer instantly saying no way.
It depends. If it is bitcoin, that is also my advice, but some altcoins are just useless and if someone invested in it, the value can depreciate, some will not get to all-time-high is another thing. If it is bitcoin, it is worth investing. As for altcoins, during bear market like this, you can gamble with it with the amount of money you can afford to lose, but you can invest in bitcoin and still have the confidence that you will see positive ROI in long term.