You can define the minimum and maximum age for the students for the admissions against the class. Follow the below path to update the age settings
Method 1:
Settings --> Admissions --> Class wise age settings --> Select Academic year, Board, Branch and click on Get --> Class wise between dates age settings --> Update the from date and to date, click on tick symbol to save
Method 2:
Settings --> Admissions --> Class wise between dates age settings --> Select the academic year --> Update the from date and to date, click on tick symbol to save