回複：請問申請綠卡時，小孩的最大年齡是多大

If it's US citizen's child, as long as the child was under 21 when the parent got citizenship, no limit.
If it's not US citizen's child or the citizenship of parent was acquired after child's 21st birthday, I-485 or CP has to be filed before the child turns 21 (new benefit according to new law passed two years ago). Better yet, approved by 21st birthday.