Age Deadline For Citizen Parent to File for Children

I received my citizenship this week. I will be starting paperwork to file for my children. My oldest son will turn 21 years old in four months. Do you think he will have to wait for a visa number to become available in order to obtain his green card?
Children of United States Citizens are protected by Sec. 2 of the Child Status Protection Act. If the petition is filed before the child turns 21, the age of the child is locked in, as of the date that the petition for classification as an immediate relative was filed by the parent. The child remains eligible as an immediate relative, after attaining age 21, as long as the child does not marry.