The insurance companies spread that cost throughout the range of policies they offer.  They could not do otherwise, or the type of bankrupcty seen by Fannie and Freddie would... umm well.  umm.. well maybe they'll just "get away with it" for a little while anyhow. As pointed out at Cap Con:

Antony Davies, an associate professor of economics at Duquesnse University, said the argument on insuring 25-year-olds has some holes in it.

"Proponents of the regulation to keep 25 year olds on their parents' insurance policies claim contradictory things. On the one hand, they claim that keeping 25 year olds on the parents' insurance won't raise insurance costs. On the other hand, they claim that we need this new law because insurance is too expensive for many 25 year olds to purchase.

Its that way BECAUSE of government intrusion.  Just this provision existing adds a way in which the insurers must underwrite risk.  It removes options, choices, and restricts flexibility to insurers in the policies they offer.

It gives them another tool to assess risk costs forward as well.  They will use as a tool, to raise rates overall, as they MUST at every step, to simply stay ahead of the "planners" in government who understand business and risk so well.  These costs get passed along in other ways

That separate plan we paid for Junior will seem CHEAP when our group plan bill comes in, and we cut the first checks that reflect the increased costs that the insurer MUST pass though or cease doing business.  Unfortunately it cannot involve the efficiencies that a free market will develop.  The new law incorporates the added cost of an over reaching bureaucratic nightmare of regulation and insistence upon unnatural business practice.  Selling insurance just became more expensive ...for the buyers.