Some of the preference categories depend on the age of the applicant.
For example, children unmarried and under 21 of US citizens are Immediate Relatives.
But if they are over 21 they fall into the 1st preference category, which has a longer wait.
Unmarried children under 21 of a lawful permanent resident are in the 2A preference category. If they are over 21, they fall into the 2B preference category.