This gist contains some info about calculating users age avoiding leap year issues.

UserAge.rb

class User
  
  attr_reader :born_on
  
  def age(today = Time.now.utc)
    return nil unless self.born_on.present? 
    ((today.to_time - self.born_on.to_time) / 1.year).to_i
  end
  
end

describe User do
  describe '#age' do
    context 'when born_on is not set' do
      before { subject.born_on = nil }
      its(:age) { should_not be }
    end

    it "calculates the user's age when born_on is set" do
      on_given_day = Date.civil(2013, 4, 30)
      subject.born_on = Date.civil(1968, 5, 1)

      subject.age(on_given_day).should eq(44)
    end

    context 'on leap year' do

      before { subject.born_on = Date.civil(1996, 2, 29) }

      it "calculates the user's age" do
        on_given_day = Date.civil(2014, 11, 4)
        subject.age(on_given_day).should eq(18)
      end

      it "calculates the user's age before birth day" do
        on_given_day = Date.civil(2012, 2, 28)
        subject.age(on_given_day).should eq(15)
      end

      it "calculates the user's age after birth day" do
        on_given_day = Date.civil(2012, 3, 1)
        subject.age(on_given_day).should eq(16)
      end

      it "calculates the user's age on birth day" do
        on_given_day = Date.civil(2012, 2, 29)
        subject.age(on_given_day).should eq(16)
      end

    end

  end
end