I can help you out with the formula:
Something like this should do the trick:
=IF(AND(C2>=5,C2<=6),"5&6",IF(AND(C2>=7,C2<=8),"7&8",IF(AND(C2>=9,C2<=10),"9&10",IF(AND(C2>=11,C2<=12),"11&12",IF(AND(C2>=13,C2<=17),"Teens")))))
As for the pivot table, you can find a lot of documentation on the web to help you out.
I would suggest adding the new column to the page section of the pivot (filter here by the age group you want), the names of the student to the column section and "sum of age" to the data section.
Once you create a pivot you can copy and paste it to the other sheets, it will save you a lot of space as they will be all using the same cache, and select in the page section the age group you want.
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