If a child is 18 can they sue one of their parents for the money that they would have had to pay for child support? If they cant sue for the full child support amount, is it possible to sue for half the amout the child had to pay for certain things?
Contrary to a common misconception, child support isn't owed, nor is it ever paid to a child.
I could ramble on with a boring legal explanation, but I'll save us both (as well as other readers) time.
If your mother or father was owed (or is still owed) child support, that's money she or he received from the other parent to assist in caring for you.
It was ordered to be paid by the non-custodial parental unit to lower the financial burden on the custodial parental unit.
To answer your question, yes, you can sue anyone you desire to sue.
If you sue your mother or father for child support, you, the plaintiff won't prevail.
You are free, however, to bring that lawsuit and see what happens.
Or, you can read this informative articles discussing variations on the question you posed:
http://family-law.freeadvice.com/family-law/child_support/back_child_support.htm
http://family-law.lawyers.com/child-support/paying-for-college-when-parents-are-divorced.html
Good luck.