If you are EB1 and I-140 is approved, your adult kid can be use child age (
CSPA age) to follow and join once your EB I-485 is approved. File I-824 at
that time. The process can easily take > 6 months.
If EB2 or EB3 and after your EB2/EB3 I-140 approval the PD never became
current (=child filed concurrent I-485 in July 2007 before I-140 approval),
you child is now aged out and can no longer get GC with you. You will have
to file F2B for child after you get GC. This will take very long (>5 years
).
Try F-1 visa (although by filing I-485 before, child should get denied if VO follows the rules)