How do you define age? Is it years, years and months or years, months and days?
Since Age is usually stated as 22 years except for very young children who like years and months...
Then year(yourdate) - year(studentsbirthday) is probably sufficient. Use an unstored calculated field to adjust for birthdays.
Let ( [ J1 = Date ( 6 ; 1 ; SchoolYear ) ;
JF = J1 - DayOfWeek ( J1 ) + 6 ] ;
SchoolYear - Year ( DateOfBirth ) - ( Date ( Day ( DateOfBirth ) ; Month ( dateOfBirth ) ; SchoolYear ) > JF )