The age of the participants should refer to the time where the voting
took place, and not the time when the budget was created.
Note that now the tests pass even when the budget is created in a year
but the balloting phase takes place in a different year. By default,
each budget phase lasts for one month, so when we create a budget in a
test, its balloting phase takes place a few months after the date when
the budget is created. Since the `between_ages` scope uses the date of
the balloting phase as a reference, and the test was using the date when
the budget was created as a reference, the test failed depending on
whether those dates took place in the same year or not.
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