I think it works in a repeat group too. But it might seem a little off 
You made me so curious, I designed the following:
Note that I’m calculating the corresponding age group inside the repeat group and summing them all outside the group.
It works like this:
You can also test it yourself in here:
https://ee.kobotoolbox.org/x/Rjvlrq5X
Also reference xlsform:
Age Count.xlsx (9.6 KB)
Best,
