Suppose you are paying an ex-wife for child support. In the US, a child becomes an adult at age 18-what then? Is support of an adult a different matter? Is adult support tax-deductable? When the child reaches the stage of self-support (i.e. having a job), is adult support still warranted? 
This depends entirely upon your jurisdiction, what your divorce decree/support order says, and how vindictive the custodial parent is.
Legally an 18 year old is an adult, yes, but in some cases child support continues until the child is out of college/21/some other fixed definition. A working child doesn’t make a whit of difference- one of my friends is continuing to pay child support on her husband’s son, who (although not yet 18) dropped out of school without his father’s permission, works full time making more than his father does, and (with the full support of his mother) refuses to abide by the visitation agreement. In all cases it is reportable on taxes as child support, regardless of the child’s age, until the divorce decree/support agreement is fulfilled.
I beleive that in most states court ordered support stops at 18 unless the child is a full time student, at which point support can be required up to the age of 21 thru 24 in some states I beleive. I know this because a former friend of mine is trying really hard to talk his intelligent daughter out of going to college just so that he can get out of child support. Bastard. :mad:
Forgot to add- anything you choose to do beyond the support order is entirely on you. If your SO runs out when your child turns 18/finishes high school but you choose to help the child financially through college, you don’t claim that money. That’s you being super nice.
Several states compel CS in the form of college tuition well beyond the age of 18, and there have been several threads on this issue. Other than the possibility of having a mentally handicapped child, I don’t think any state requires non-tuition CS beyond the age of 18. I think there are situations where a parent with an adult child (or the adult child themself) can sue for recovery of unpaid child support from a non-paying parent many years after the child turn 18.
Ummm… well I guess you could “report it”, but unless something has changed CS is not tax deductible in any fashion that I’m aware of.
It’s not deductible, no, but when we were paying on Mr. Kitty’s kids (several years ago now) there’s a section where you report child support paid. I always thought it was to link up whether the custodial parents were reporting the full amount received on their own taxes.