Can you build a scope by model method value? (Edited showing non-working scope vs. working scope)

user.rb

class User < ActiveRecord::Base
  # ...
  # Old scope: scope :adult_purchases_enabled, -> { where(self.age >= 21) }
  # Working scope:
  scope :adult_purchases_enabled, -> { where('birth_date <= ?', Time.current.advance(years: -21)) }
  # ...
  
  # Public Methods
  def age
    (Time.now.utc().to_s(:number).to_i - self.birth_date.to_time.utc().to_s(:number).to_i)/10e9.to_i
  end
end