An example of how to decouple the interesting behaviour out from a user interface

interface.rb

require_relative 'users'

users = Users.new

loop do
  puts "---------------------------------"
  puts "Menu:"
  puts "1. Add a user"
  puts "2. List users"
  puts "3. Sort users by name A to Z"
  puts "4. Sort users by name Z to A"
  puts "5. Sort users by age young to old"
  puts "6. Sort users by age old to young"
  puts "7. Quit"
  print "> "
  case gets.chomp
  when "1"
    print "Enter the user's name: "
    name = gets.chomp

    print "Enter the user's age: "
    age = gets.to_i

    users.add(name, age)
  when "2"
    puts "---------------------------------"
    puts users
    gets
  when "3"
    users.sort_by_name_asc
  when "4"
    users.sort_by_name_desc
  when "5"
    users.sort_by_age_asc
  when "6"
    users.sort_by_age_desc
  when "7", "exit", "quit"
    break
  else
    puts "Invalid input, please enter a number 1 - 7"
    gets
  end
end

users.rb

class User
  def initialize(name, age)
    @name = name
    @age  = age
  end

  def name
    @name
  end

  def age
    @age
  end

  def inspect
    "#<#{@name} (#{@age})>"
  end
end


class Users
  def initialize(users=[])
    @users = users
  end

  def add(name, age)
    @users << User.new(name, age)
  end

  def sort_by_name_asc
    @users = @users.sort_by { |user| user.name }
  end

  def sort_by_name_desc
    @users = sort_by_name_asc.reverse
  end

  def sort_by_age_asc
    @users = @users.sort_by { |user| user.age }
  end

  def sort_by_age_desc
    @users = sort_by_age_asc.reverse
  end

  def to_s
    name_length = (largest_name || "NAME").length
    age_length  = (largest_age  || "AGE").to_s.length
    string      = ""

    header = "NAME".ljust(name_length) + " " + "AGE".ljust(age_length)
    string << header << "\n"

    divider = "=" * header.length
    string << divider << "\n"

    @users.each do |user|
      user_string = user.name.ljust(name_length) + " " + user.age.to_s.ljust(age_length)
      string << user_string << "\n"
    end

    string
  end

  def largest_name
    @users.map { |user| user.name }.max_by { |name| name.length }
  end

  def largest_age
    @users.map { |user| user.age }.max
  end
end

users = Users.new
users.add('Anita',     5)
users.add('Britney',  20)
users.add('Chad',    103)
users.sort_by_name_asc  # => [#<Anita (5)>, #<Britney (20)>, #<Chad (103)>]
users.sort_by_name_desc # => [#<Chad (103)>, #<Britney (20)>, #<Anita (5)>]
users.sort_by_age_asc   # => [#<Anita (5)>, #<Britney (20)>, #<Chad (103)>]
users.sort_by_age_desc  # => [#<Chad (103)>, #<Britney (20)>, #<Anita (5)>]
users.to_s
# => "NAME    AGE\n" +
#    "===========\n" +
#    "Chad    103\n" +
#    "Britney 20 \n" +
#    "Anita   5  \n"