Will kid's I-485 be approved if he's been over 21-year old? - 回複

I'm not sure but I believe it may not be a strait "yes" answer.

When browsing I-485 SOP, I glanced USCIS has a policy "to expedite the processing of the I-485 for dependent
children who are within one year of reaching the age of 21" (P 47 of 269 - Age-Outs) which implies that it would be issue for a dependent who has not granted GC prior to 21 year birthday.