Not all in because they have also a life so they want to buy maybe games with their money. Let them invest a little bit so that they can have more money in the future.
They can than use this money to afford things which they couldn't afford when they were younger.
Yeah you're right. They dont have to risk all of their money just for them to be rich in the future. They should know first the risk involve in investing in bitcoins. As @Britegrafter said above, there are also some usefull things that they can do with their money.