users_by_birthyears_.rb

birthyears_users = {}

User.unscoped.registered.select([:id, :birthday]).find_each do |user|
  if user.birthday
    birthyears_users[user.birthday.year] ||= []
    birthyears_users[user.birthday.year] << user.id
  else
    birthyears_users[0] ||= []
    birthyears_users[0] << user.id
  end
end; nil

birthyears_users.sort.each do |year, user_ids|
  puts "#{year}\t#{user_ids.count}"
end; nil

users_by_birthyears_in_taiwan.rb

birthyears_users = {}

User.unscoped.where(service_zone_id: 'taiwan').registered.select([:id, :birthday]).find_each do |user|
  if user.birthday
    birthyears_users[user.birthday.year] ||= []
    birthyears_users[user.birthday.year] << user.id
  else
    birthyears_users[0] ||= []
    birthyears_users[0] << user.id
  end
end; nil

birthyears_users.sort.each do |year, user_ids|
  puts "#{year}\t#{user_ids.count}"
end; nil