Until what age do I have to pay child support in Pennsylvania?

​"Blue, you're my boy!​" -- Will Ferrell in Old School

Here's the answer to your pregunta, straight from the case law of Pennsylvania:

Notwithstanding a child reaching majority at age 18, a parental duty of support is owed until a child reaches 18 or graduates from high school, whichever event occurs later. Blue v. Blue, 532 Pa. 521, 616 A.2d 628 (Pa. 1992).

Now, don't get all blue (and certainly, please don't get all puffed up and blue) because you thought your child support obligation ended at 18 years of age. Just hope that your child is a prodigy and graduates from high school early so that you can look in the mirror some day and say with a smile on your face, "gosh darn it, I was right!"