The age limit for a K-2 to file his AOS application is his 21st birthday. However, some USCIS offices interpret this (more restrictively) to mean that he must be under 21 at the time of the interview. Your wife's child is always your step-child. But if you marry before the child REACHES 18, then he would be considered your child for immigration purposes, and you could file an I-130 which would preserve his status.