age.rb

class Age
  def self.for_date_of_birth(dob, today = Date.today)
    # http://stackoverflow.com/a/11942/445124
    diff = today.strftime("%Y%m%d").to_i - dob.strftime("%Y%m%d").to_i
    diff / 10000
  end
end

age_spec.rb

require_relative "../../app/models/age"

describe Age do
  it "calculates a person's age given their birth date and the current date" do
    expect(Age.for_date_of_birth(Date.new(1980, 7, 3), Date.new(2008, 8, 14))).to eql 28
  end

  it "calculates a person's age who was born on a leap year February 29" do
    expect(Age.for_date_of_birth(Date.new(2000, 2, 29), Date.new(2009, 2, 28))).to eql 8
  end
end