Although children become adults at age 18, this doesn't necessarily mean that their dependency upon their parents suddenly stops. Typically, child support stops when a child reaches the age of majority, but child support law varies by state. Some states do allow child support to continue past age 18 and graduation from high school. Whether or not you can receive support for your child after she reaches age 18 depends upon where you live.
Some states terminate child support at the age of emancipation -- typically 18 years -- regardless of whether the child is attending college either full or part time. Even in these states, however, if parents have a marital settlement agreement that provides for support past the age of emancipation, courts will generally enforce the terms of the agreement. In states where courts are without the power to order parents to pay child support past the child's emancipation or contribute to the expenses of college-aged students, agreeing to take on these obligations can be powerful points of negotiation.
In states that allow child support to continue past the age of emancipation, it is usually conditioned upon the child continuing her education past high school. It also usually depends upon the student pursuing a full-time course load. Some states go even further than requiring full-time enrollment; in these jurisdictions, students must be clearly pursuing a degree and maintaining decent grades. Unrelated credits or failing grades in multiple courses may get the student's parents off the hook.
Some states terminate child support -- direct support payments to a custodial parent -- at emancipation but require both parents to contribute to college tuition and costs. In general, states don't require parents to pay college costs in their entirety; the student retains some degree of responsibility by contributing to her own education through student loans, work-study or grants. Also, the parent's ability to pay may become relevant in some cases. If the other parent takes you to court on contempt charges for failing to pay, she'll generally have to show that you could have paid but simply chose not to do so.
To Contribute or Not
In states that require no support beyond the age of emancipation, parents can agree to contribute to or entirely pay college expenses in a marital settlement agreement. While this can be an important negotiating point, parents of young children must balance their desire to provide for their children's academic futures against the possibility that they may not have the financial wherewithal to contribute when their children finally reach college age. Since obligations assumed in a contract are usually not modifiable by courts, a parent could be saddled with a crippling obligation she can't afford.