First, letting the child stay in the parent's insurance doesn't mean the parent has to pay for it, that's between the parent and the child -- the parent *can* say, "I'll keep you on my health insurance, but you have to pay the marginal cost" -- that's between the parent and the children. It's acutarially sound because it keeps a lot of healthy young people, and their premiums, in an insurance pool.
But that parent CAN'T say "I can't/won't, you have to get your own."
Ever co-sign a loan and have someone back out of their end of the deal?
At age 70, health insurance is more expensive than ever, yet earnings are at an all time low for most people. Still, those grandparents are on the hook, whether or not the child pays in a dime.
It doesn't keep the young people in the insurance pool, but it will eliminate the assets of those who raised them. Already, just a few years from drawing SS, my projected income from social security would not cover the current cost of the cheapest plan in my state for a family of 4, and that based on current ages, not by the time the grandkids are 26.
For the forward looking, that means grandparents will be economically forced to abandon grandchildren to the tender mercies of the State to prevent this situation. Yet another rift in the extended family, and yet another gap which will ensure that a generation which could provide wisdom and perspective for a younger generation is prevented from doing so--exactly what the totalitarians want, because children not regaled with memories of a much freer existence will never crave a (relative) Liberty they never knew.
All of this crap has far deeper implications than just money, that is only the blunt instrument by which policy will be imposed.