user_category.rb

require 'benchmark'

class AgeGroup < Struct.new(:categories)
  # def 
  #   Hash[*AGE_CATEGORIES.keys
  #         .map { |r| r.map { |v| 
  #           [v, AGE_CATEGORIES.select 
  #             { |k, val| 
  #               k.include?(v)
  #             }.values.first]
  #         } }.flatten].freeze
  # end

  def calculate_table
    Hash[*categories.keys.map{ |r| expand_range(r) }.flatten]
  end

  def expand_range(range)
    range.map { |n| make_age_array(n) }
  end

  def make_age_array(age)
    [age, find_match_for(age)]
  end

  def find_match_for(age)
    categories.detect { |k, val| k.include?(age) }.last
  end
end

class User < Struct.new(:age)
  AGE_CATEGORIES = { (0..9) => :child, (10..18) => :teenager, 
                     (19..28) => :young, (29..50) => :adult, 
                     (50..120) => :senoir }.freeze

  COMPUTED_CATEGORIES = AgeGroup.new(AGE_CATEGORIES).calculate_table


  def if_category
    age = self.age
    if age < 10 then :child
    elsif age < 18 then :teenager
    elsif age < 24 then :young
    elsif age < 54 then :adult
    elsif age < 100 then :senior
    else :deity
    end
  end

  def case_category
    case age
    when 0..9 then :child
    when 10..18 then :teenager
    when 19..28 then :young
    when 29..50 then :adult
    when 50..100 then :senoir
    else :deity
    end
  end

  def lookup_category_uncached
    age = self.age
    AGE_CATEGORIES.detect { |k, v| k.include?(age) }.last
  end

  def lookup_cached
    COMPUTED_CATEGORIES[age]
  end
end

Benchmark.bm(15) do |x|
  x.report("if") { 
    10000000.times do 
      user = User.new(Random.rand(100))
      user.if_category
    end
  }

  x.report("case") {
    10000000.times do 
      user = User.new(Random.rand(100))
      user.case_category
    end
  }

  x.report("lookup") {
    10000000.times do 
      user = User.new(Random.rand(100))
      user.lookup_category_uncached
    end
  }

  x.report("lookup_cached") {
    10000000.times do 
      user = User.new(Random.rand(100))
      user.lookup_cached
    end
  }
end

Yeah I was just discussin this with @scomma