They all get it, they (almost) all recover, they are then no longer carriers of it and can go and help the over-50s. I'm not saying that's a workable plan, but I think that's the logic.
And all the over 50s in multi-generational households then get it too. It's a very stupid and callous 'plan', so I'm sticking to only pointing out the most glaring problems with it.
What problem do you think you're solving by deliberately allowing all the under 50s to get infected?