Is there a variable that displays at what age the children enrolls in a certain grade?
There is not a variable in IPUMS-CPS that directly reports the age a respondent entered a grade level. You might be interested in the Education Supplement variables EDGRADE and EDGRDLY. The former reports the respondent’s current grade level and the latter reports the respondent’s grade level one year ago. This will let you know if a respondent is repeating a grade level or if it is the child’s first year in a grade level. You can use this information along with AGE to estimate the age a child enrolled in a certain grade.
Hope this helps.