A couple of things:

First, as Daniel already said, why not put the information right in the
loop? However, don't go recursive there, I'd do a loop instead:

age = 0
target_age = 30..65 # ;)
while ! target === age
  print "Please enter your age: (as two digits): "
  result = gets
  age = result.chomp.to_i
  case age
  when age < 30
    puts 'Sorry, too young.'
  when age > 75
     puts 'Sorry, too old.'
  else
     puts 'Hava a nice holiday!'
     return age
  end
  puts "Remember, age must be entered as two digits, e.g. 35"
end

check_age