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.
Just read a topic here about foolish thing he did. He invest all the money she and his wife has and now they are on their to become rich couple. Though of course, since young people have many things to consider, being all in is not a good option. Just invest the money you are willing to risk with.
One more thing young people will not show interest in this kind of business in early age, at this age they want only enetratinment. So investing all money in bitcoins is not worth for young people.