Yes, the child must be 21. Additional point to consider is that those who need a waiver (who can't adjust in the us) must show extreme hardship to a qualifying relative. The provisional waiver program considers only us citizen spouses or parents to be qualifying relatives. So, even if your child could sponsor you, you don't seem to have a qualifying relative under the provisional waiver program.
The only time you can use a child as a qualifying relative is if the reason for your inadmissibility is criminally related (and you would have to pursue a refular I-601 waiver not the provisional waiver if that is the case since only persons whose SOLE ground of inadmissibility is unlawful presence are eligible for the provisional waiver.
The information offered is general in nature and not meant to be relied upon as legal advice. Please consult an attorney prior to making legal decisions. Visit us at www.tunitskylaw.com. Contact us at 713.335.5505 or email at email@example.com. Veronica Tunitsky offers in-person, as well as telephone and email consultations.