Here is the problem with what you are asking: what happens when we spen time making a solution, and three months from now you want it to do something a little more? Wouldnt it be best if you learned how to make these things for yourself?
I would suggest using a database, that way you can run a report on the data to figure out the age groups. It is quite simple in a database.
Name birthdate gender favoritecolor
Group one report
Select all children from child table that have birthdate between 2011-2014
Group two report
Select all children from chikd table that have birthdates between 2015-2018
Select all girls from child tablebthat have a foavorite color of red.
The last one is to showbhow your child record can scale based on your needs in the near future. With using a database, you can make changes to the data, without disrupting the program, as there is no codeing involved, jist data entry amd report writing.
Let me know if you need help with a database solution. Once again, I will not provide a turn key solution, but will guide you so that you are in control of your application and you can make changes as your needs change!