Expose age_range instead of date_of_birth for Voter/User class

spec/lib/age_range_calculator_spec.rb

@@ -0,0 +1,14 @@
+require 'rails_helper'
+
+describe AgeRangeCalculator do
+  subject { AgeRangeCalculator }
+
+  describe '::range_from_birthday' do
+    it 'returns the age range' do
+      expect(subject::range_from_birthday(Time.current - 1.year)).to eq(nil)
+      expect(subject::range_from_birthday(Time.current - 26.year)).to eq(26..40)
+      expect(subject::range_from_birthday(Time.current - 60.year)).to eq(41..60)
+      expect(subject::range_from_birthday(Time.current - 200.year)).to eq(61..subject::MAX_AGE)
+    end
+  end
+end