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

I think you're measuring different things. if_category creates a lot of string objects while lookup_cached returns symbols. Also you should create a temp variable for age in the if_category speed things up.

  def if_category
    age = self.age
    if age < 10 then :child
    elsif age < 18 then :teenager
    elsif age < 28 then :young
    elsif age < 50 then :adult
    elsif age < 100 then :senor
    else 
      :deity
    end
  end

lookup_category_uncached does too much. Could be even more optimized if changing the categories hash into an array, so detect doesn't create an intermediate array. This makes it twice as fast.

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

Yeah I was just discussin this with @scomma