Try to use: GetAsNumber( Age ) into your script.
Sounds like your age is being calculated as text rather than number. As text, "9" is greater than "18" because the comparisons will follow the same rules you would use to sort words into alphabetical order.
As raybaudi suggests, getAsNumber ( age ) is one way to force the calculation into number form so that the comparisons work correctly. You can also edit your original calculation so that it returns its value as a number instead of text.