error

Failure/Error: it {should validate_presence_of(:last_name)}
NoMethodError:
undefined method `>' for nil:NilClass

user.rb

class User < ActiveRecord::Base
  devise :database_authenticatable, :registerable,
    :recoverable, :rememberable, :trackable, :validatable
  validates :first_name, :last_name, :dob, presence: true
  validate :check_age, on: :create


  def full_name
    [first_name, last_name].join(' ')
  end
  protected

  def check_age
    if dob > Time.now - 20.years
      flash[:notice] = 'Too Young'
      false
    else
      true
    end
  end
end

user_spec.rb

require 'rails_helper'

describe User do
  it {should validate_presence_of(:first_name)}
  it {should validate_presence_of(:last_name)}
  it {should validate_presence_of(:dob)}
  it {should respond_to :full_name}
end

I'm trying to have my User invalidate if user is too young (< 20). but when rspec checks, for example, that the last_name is present, it causes my custom validator method to run, and since dob is nil at that point, it crashes thusly.
Any advice?