In the State of California there is a rarely used Family Code that states the "The father and mother have an equal responsibility to maintain, to the extent of their ability, a child of whatever age who is incapacitated from earning a living and without sufficient means."
I recently used this law to win continued child support for both of my 18 & 20 year old children with mental illnesses.
Research on this law could be used to get it in place in other states. Here's the full story. http://atwitsend.org/fighting-good-fight/